It means different specific things to different religious people but usually it means using "lord" or "god" as part of some expletive, which is regarded as disrespectful.

On the other hand some religious people can say "god damn it" without a trace of shame and I don't know why not. God could make himself useful by damning some things. All it means basically is "I wish god would banish this nuisance to the pit" and it's almost always something that people could do without, like stubbed toes or smashed fingers. Other times it's just an ironic utterance.

It's only a big deal for people who decide to make it one ... and who understand that their god is an immature and easily offended child.

Also I'm not sure but what the original author of that particular prohibition had something more specific in mind than the above. Vowing in the name of god to do something and failing to follow through for example. As in "I swear to god I'm telling the truth" and actually lying. This would be a more specific and practical harm that would be a little more worth prohibiting in a religious context.

Those who care to parse the original Hebrew sometimes conclude that this is a prohibition against perjury, or against invoking god in the service of magic. Or simply against using it in a negative context -- the name should be invoked only in blessing, or perhaps only in actual worship of the deity.

To use it lightly, without the respect it deserves.