I have a sweater that says to machine wash and then “block flat to dry”. What does that mean?
It just means don’t crumple it up into a ball or put it on a hangar. Lay it out flat to dry, on a drying rack if you have one.
It means to lie it flat on a towel or something else to dry, and smooth it out to its normal shape and leave it there until dry.
Knitted and other non-woven fabric items are often supposed to be “blocked”, i.e., laid out flat while wet in their desired shape and size and left to dry that way.
This is because they tend to be stretchier than most woven fabrics, and can stretch or twist out of shape if left hanging.
Yeah, I made the mistake of hanging a heavy woollen sweater up after handwashing it. It ended up skin-tight and about knee length. 
Luckily, if you make that mistake, re-wetting and re-blocking will usually return it to its normal shape.
Blocking is a little more than just laying flat - it means lay it flat, stretch/shove it into the final shape you want it to take, THEN let it dry.
You are supposed to lay the garment out on a flat surface, mark the outline, wash it, and then lay it on that flat surface again and push and pull and pat the garment into the original shape. I never do this, but that’s how you’re supposed to do it. Ideally, you do this on a towel or on a sweater dryer.
I avoid doing this sort of thing by trying to never buy anything that is NOT machine washable and dryable. If someone gives me something that requires this sort of care, I generally send it to the dry cleaners, at least until my husband does the laundry. He washes everything in hot water, and then he throws it in the dryer at the highest setting, for at least 90 minutes. This works fine for his jeans and polo shirts. It doesn’t work so well for my nice knitted garments.
Wow, I thought my dad was the only one to do laundry that way. I started doing my own just so he’d stop ruining my stuff, not because I had to do it as a “chore”. He even ruined a few of my towels that way. Friggin towels.