Blocking is when you wash/moisten your knit or crochet and lay it down to dry to its final form.
My Grandma recently knitted my daughter a sweater and sent me a photo of this process. I had no idea this was part of knitting. Your sweater looks great, btw!
It’s not absolutely necessary but one of those little things you can do to make the knit look really polished. Has your grandma taught you how to knit?