English slang, two people dating, not in a relationship.

To "Check-in" is in a sense, the opposite of "checking someone out."

When a person uses an app on their smartphone (or iPad) to "check-in" at a business, destination, or popular point of interest. If a person checks-in, it usually lets other people know where they're at via a social media channel such as Yelp, Foursquare, Facebook, and in some cases Twitter.

Same as trying to get to know a person and what they are about.

Roast battle in Memphis slag is checkin

When a fuckboy periodically txt you or call you every other day or week to check in on how you are doing and if he feels that he is losing you to someone else, he will proceed to ask a bunch of questions to draw you back to him

to be an item with someone so that they are your boyfriend or girlfriend.