This phrase, "Call it a day", means to stop what you're doing for that day.
You use this when you're not 100% finished with everything you need to do, but you're tired or ready to go home. Use "Call it a day" when you're working on projects with people, practicing for something together, or just to let people know when you're leaving work:
Jean, I'm going to call it a day. I'll see you tomorrow.