What it is:

https://en.m.wikipedia.org/wiki/Telegram_(software)

Signal may be comparable (improved security in some respects, not something I'm very bothered about) but I haven't got a way of running it.

If you try looking for useful answers to my question via search engines, you tend to find they're drowned out by hucksters trying to make a fast buck. What's in it for Mudcat users generating mindless destructive noise about it I can't imagine.