Meaning - to go to bed in order to sleep
This expression is used when you want to tell someone that you are ready to go to bed in order to fall asleep.
I had a long and busy day at work yesterday. So, I hit the sack as soon as I got home last night. I am glad I got enough sleep because I feel much better today.
John: Hey! You are home. How was school?
Lisa: It was fun and we practiced a lot of new words. But now, I am so tired. I think I am just going to hit the sack.
John: Already? Aren't you going to study for your test?
Lisa: I don't think I can. I am just so sleepy. I will wake up early and study tomorrow morning.
John: OK, good luck!