Floating content within website?

hello all

im VERY sorry but i dont know what its called but, what im trying to do is, you know how sometimes youll go to a website and when you click on say a photo or a video, the background of the website goes semi dark and then the content shows up and starts playing.

what is that called and/or can someone point me in the right direction?

thanks and sorry if i put this in the worng place.