To check in means to report your arrival or confirm your presence at a place. When your family arrives at a hotel, you check in at the front desk to get your room key and let them know you're there. At an airport, passengers check in before their flight to confirm they're taking the trip and to get their boarding passes.

The phrase also means making contact with someone to see how they're doing. A teacher might check in with a student who seems upset, asking if everything's okay. Friends check in with each other when they haven't talked in a while. Parents might check in by text when you're at a friend's house, just to make sure things are going well.

Checking in shows responsibility and consideration. When you check in with your parents after school, you're letting them know you're safe. When a coach checks in with team members during practice, they're making sure everyone feels included and ready to play. The phrase suggests a moment of connection or confirmation: a brief “I'm here” or “How are you doing?” that keeps everyone informed and connected.