"Check in about" is a common phrase to ask for an update or status on something (a project, person, situation) in a friendly way, often via email or message, meaning "to inquire about the progress/well-being of". It's a phrasal verb (check in) followed by the preposition "about". You're looking for information or an update, like asking "How's that report going?" but phrased as "Just checking in about the report".