It's a digital mode that has impressive ability to hear weak signals. Which means you can have a small, inexpensive radio, a really suboptimal antenna, and still make worldwide contacts.

The basic mode only allows you to make contact, it doesn't allow for discussions or data transfer. It's simply a “I'm here, and I can hear you” mode.

It's controversial because a) to some hams, conversation is the reason to be on the air; and/or b) hams can be opinionated and grumpy, and some feel like if you haven't spent $5k on your radio/built your radio from scratch/exclusively use Morse code/devoted five acres of land to a 200 foot tall tower with a Yagi antenna you aren't doing it right. DX (long-distance communications) is an exclusive club. FT8 makes it a lot less exclusive. There's always resistance to that.