The bathtub drain is commonly clogged by hair and other debris. However if you have a bathtub with a trip lever drain set up the trip lever could be stuck in the down position, but most likely your bathtub drain is clogged with something.
You could first try use a plunger to clear the tub and see if that works. If the plunger fails you the next thing you will want to do is to look into the tub drain and see if you can see any hair, soap chips or anything else that can be clogging the drain. If the drain looks clear to the naked eye then you have one last solution before having to call in the plumber.
If you have tried both of the above methods to unclog your drain and you are still not successful you can give a liquid plumber a try and see if that helps clear your clog. If the liquid plumber does not work, then at this point it’s best to call a licensed plumber to bring in a drain cleaning machine to clear your lines.
One last other thing is if you are living in basement apartment and you have a clogged bathroom drain and the above three steps have not worked and you just rent the apartment you might want to talk to your landlord. In some cases the plumbing system might be dumping into a septic system a.k.a. a cesspool and the cesspool just might have to be pumped out, so that is one last possibility as well.