It means you live in the moment. For others it can mean your existence is defined by what you do or that you ultimately decide what the meaning of your life.

We accept (embrace) 100% responsibility for our life and circumstances. We are not victims. We blame no one, hold no anger or grudges over the state of our lives.

"Existence precedes essence"

In short: what we do determines what we do.
It makes perfect sense

Have you ever played the "Game of Existentialism"?

Winning: You are not currently thinking about the "Game of Existentialism" in any way.
Losing: You are currently thinking about the "Game of Existentialism" in any way.

Congratulations and welcome to the game!! There is no leaving the game, the game is not optional and you cannot choose not to play. Now that you know of the game, you are a participant for the remainder of your life. Tell your friends so they can join in.

Status: You are currently [LOSING]

What does it mean to you?

It means acceptance of reality, of the fact that life and existence has no inherent meaning. It meas that we are free to define our own meaning, to make our own choices, decisions, and actions and to be responsible for them.

To choose to be in disagreement with Predeterminism

To me, it means absolutely nothing.
I don't have a whole lot of use for most philosophy.
I find it gets in the way of actually dealing with the business of life.
What other people want to do, or think, is entirely up to them.
If immersing oneself in the various (and often contradictory) philosophies espoused by those who believe they "know better" works for you, have at it.

Sartre distinguishes the pour soi from the en soi, Heidegger uses the terms sein and da sein. This casts a distinction between things that exist as they are, and those that choose what they will become. Human beings are in the latter group. Other phenomena in the former. A rock is a rock. A bird is a bird. A human can be whatever the sum of his or her choices allow. That is a very short look at a complicated philosophical set of theses and proofs.

Existence precedes essence. To me, that means we show up on the scene through natural processes, and then we create meaning in our lives through our choices.

Existentialism = Existence precedes essence in human beings (Sartre’s definition)
Essentialism = Essence precedes existence in human beings

For example, what the sapling will become is already there in the acorn; it has to become an oak and cannot choose to become a pine or poplar. Likewise, what type of bird will hatch is already there in the fertilized egg, and the same for plants and animals universally—even for human beings according to the essentialist. For example, Aristotle contended that the human essence is rationality. Hence the definition, “Man is a rational animal.”

But existentialists contend the human beings come into existence without any essence to predetermine what they will become. Instead, they have free will, and therefore must choose what type of creature they will be. They may choose to become rational or not; they may choose to be gregarious or loners, truthful or dishonest, etc. for they (=we) have no essence that predetermines this.

The implications of this view for ethics, logic, and theory of knowledge are quite far-reaching.

Authenticity (philosophy)
From Wikipedia, the free encyclopedia
Authenticity is a concept in psychology (in particular existential psychiatry) as well as existentialist philosophy and aesthetics (in regard to various arts and musical genres). In existentialism, authenticity is the degree to which an individual's actions are congruent with their beliefs and desires, despite external pressures; the conscious self is seen as coming to terms with being in a material world and with encountering external forces, pressures, and influences which are very different from, and other than, itself. A lack of authenticity is considered in existentialism to be bad faith.[2] The call of authenticity resonates with the famous instruction by the Oracle of Delphi, “Know thyself.” But authenticity extends this message: "Don’t merely know thyself – be thyself."[3]

You’re smoking some good weed !!

"Existentialism" definition by dictionary.com:

noun

Philosophy

A philosophical attitude associated especially with Heidegger, Jaspers, Marcel, and Sartre, and opposed to rationalism and empiricism, that stresses the individual's unique position as a self-determining agent responsible for the authenticity of his or her choices.

Contemporary examples of existentialist

In the grips of existentialist angst, investors decided to sell stocks and start stowing money under their mattresses.

British Dictionary definitions for existentialist

A modern philosophical movement stressing the importance of personal experience and responsibility and the demands that they make on the individual, who is seen as a free agent in a deterministic and seemingly meaningless universe.

Existentialist in Culture

A movement in twentieth-century literature and philosophy, with some forerunners in earlier centuries. Existentialism stresses that people are entirely free and therefore responsible for what they make of themselves. With this responsibility comes a profound anguish or dread.

Søren Kierkegaard and Feodor Dostoyevsky in the nineteenth century, and Jean-Paul Sartre, Martin Heidegger, and Albert Camus in the twentieth century, were existentialist writers.

No one can tell you what it means to be an existentialist, you need to discover that for yourself.