drawing over mask (??)

I have no idea if this is gonna make sense or not, but when i open a masked layer and i draw over it, it just goes over the mask ? Maybe its supposed to do that, but i saw a video and it was different, like, it didnt draw over the mask,,, so how do i do that?? im trying to redraw some stuff except trying to make sure that it doesnt draw over the layer is kind of annoying ... this is an example i really hope this makes sense fjhsjdk...  like isnt it supposed to cut off where the mask ends??... help