It's a messaging service like Discord, except it links to your phone number. It's pretty cool, has chat rooms and groups, although I find Discord to be a little more useful for grouping with other people (using voice chat, video calls, etc.). Still, some furs only use Telegram, so it definitely comes in handy.

as i underrstand its mostly that you can upload a bunch of giant emotes featuring your own fursona to post at people