Graphics.Blit

Name	Description
source	Source texture.
dest	Destination RenderTexture, or null to blit directly to screen.
mat	Material to use for copying. Material's shader could do some post-processing effect, for example.
pass	If -1 (default), draws all passes in the material. Otherwise, draws given pass only.

Copies source texture into destination render texture.

This is mostly used for implementing image effects.

Blit sets dest to be active render texture, sets source as _MainTex property on the material, and draws a full-screen quad.