i see many men here saying they are looking for a dominant woman or want to be someones slave. and i have tried asking what they mean still don't understand it lol.

They want you (the woman) to take charge of all sexual activities, When, Where and What specifically.
Some guys who want a dominant woman also want her to tell them what to do in all aspects of the relationship.
To those guys, I say grow a pair!

It can go further than that. Slave / Master or slave / Mistress is often 24/7. Not always, but can be. So not just sex, but whenever they're home.

It can take any shape or form, just what both parties of the couple want and agree upon. Example could be the Master or Mistress sitting on a settee, the slave at the floor, maybe on a cushion.
Other like to have a collar with chain that's held by the Master/Mistress when they go to a BDSM event and so on.

I think on dating sites, however, people mostly mean dominant in bed. Being told what to do, made to do things, orgasm control, being tied to the bed, blindfolded, spanked, that kind of thing.
Again, how far that goes depends on what both want & need.