People usually wet or steam the finished project, stretch it a bit and then pin or clip it to a flat board.
You know how if you wash a sweater and then hang it on a hanger? It will have hanger marks in the shoulder and be all wonky. Yarn adjusts. It is just to make it adjust to the shape you want.
It won't stop stockinette from curling but it is good to do with sweater peices before putting together as they will be more flat.
It is "setting" the knitting. People don't tend to knit absolutely perfectly but part of the purpose blocking is to even that out a bit.