Common English Expression

-keep in touch

You're grabbing a

coffee at a local coffee shop and you see someone that you went to college with. It's been almost 5 years since you last saw them. You start a conversation with them and after an hour of talking, you exchange phone numbers.
As you're leaving the cafe to go home, your old classmate says, "Hey, please 'keep in touch.' I really enjoyed our conversation and I missed talking to you."
So, your old classmate wants to make sure that you continue talking with them in the future.