Does anyone know what rez these are?

Ive been getting into texture packs lately and i more than not go for the 1 k(Not sure what these numbers mean)

I believe default skyrim is 512 and the high res packs are 1024.

That number refers to the resolution, in pixels, of the textures. 1k textures 1024x1024 pixels. More pixels, more quality/detail.