Could be anything. I assume you are worried it's a dating app and afraid to ask the person who sent the screenshot, because that is the usual story (I'm afraid we get these queries most days, and 49 times out of 50 that's the reason). But hearts are used to denote "likes" in many apps, by fitness apps (unlikely to post that many notifications, but who knows?), and many others you wouldn't expect, so it could be many things.
There are unfortunately also a vast number of apps that use such notifications, so you will be lucky if someone recognises this particular one. It may be simpler to ask the person.
As an aside, if it is incriminating they would have to be extremely stupid to send a screenshot with that many of them in the picture...