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.
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.
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.