It's a Mastodon client. It has many neat features. I like it on desktop because it's easy to navigate with my keyboard. It's also great on phone. Overall pleasant, Mastodon users should try it out. But I feel like describing it at length is not really productive - it's a user interface.
If you don't want to trust people with your account details that's fine - then using a third party app is probably not for you, unless you're willing to either trust people or dig into the source code.
I’m not giving access to my Mastodon account to some random service I’ve never heard of for no reason.
If it makes you feel better, it's all client-side: there's nothing executing on the server (I'm running a copy of it on a server that just... can't execute anything) so it's not doing any data stealing.
Buuut, since it's trivial to host, you could grab a copy of the code and host it yourself as well.
This person couldn't be bothered to search "Phanpy Mastodon" to learn more, literally everything is non-trivial to them. They probably have to remember to breathe.