Film grain. Photographic film is an emulsion of silver crystals, and those crystals are not evenly sized or evenly spread, so a developed photograph has a fine random texture in it. Digital images do not, and often look sterile without it.
The grain here is random noise generated fresh at the resolution you are rendering, one value per pixel, so it is as fine as the output allows. Two choices make it convincing rather than harsh. It is sampled smoothly rather than sharply, which gives each speck a slightly soft edge the way real silver does instead of a hard salt-and-pepper dot. And it scales with the output size, so a print-resolution export keeps grain you can actually see rather than shrinking it to nothing.
alpha sets how much shows. pause freezes it, which is the difference between a still photograph and grain that boils distractingly on every frame.