Our shower drain goes down in just a large hole under the bathtub where you would think a drain would be. This large hole is half full of water and when you use the bathtub and drain it the water fills the hole and then starts to leak from under the tub. We have a small access door on the side side of the wall from the kitchen that look into the drain side of the shower. What is this called and should we clean it out?

Is this strictly a shower, or is it a tub shower. Either way, the hole that the water is going into isn't right. Sounds like you have a leaky or disconnected drain, and possibly a plugged drain. Get all of the water out of the hole and tell us what you see(as far as pipes go). A pic would help also.

The hole under the tub is there on purpose. It was left there when the slab was poured. It houses tub trap. If it fills up than there must be disconnected (... and maybe even clogged ) drain. Take flashlight and inspect the pipe under the tub inside the hole. You can fill up tub and drain while you're inspecting it so you can see where is the water coming from.

That "hole" is called The Tub Dap-Out" and is normal for a tub installation . What isn't normal is to have any water in it. Are you sayingthe shower drains into it without being connected to anything? Let me know. Tom

I should have clarified, the hole is there for the reasons Milo and Tom explained, just the water in the hole isn't right.