He is on a Dating App
Generally, someone who has an active dating app has their door open to meeting other people, either hoping someday a person they really would like would pass through that door or hoping they could get some side action. If your guy is active on a dating app, he's basically telling himself, "Let me see if I can find someone better" or "Let me see if I can get some side action."
There is an exception, however. If you just started dating, and you haven't given him an indication that you're dating only him, that you're interested in only him, then him being active on a dating app doesn't necessarily mean he's not into you. It could be that he just doesn't want to put all his eggs in one basket yet. If you're dating only him and want him to do the same, then be honest with him about it.
Tip: If you’ve met on at dating app/site, you can check every now and then to see if any of his pictures or bio has changed. Any change is an indication that he’s still active on the site.