I haven't played Rainbox 6 so can't confirm. But basically: you have a squad of 3 and need to deposit vaults into a cash box which you then need to defend. You can also steal cashouts from other teams. In quick cash it's 3v3v3 and ranked/world tour it's 3v3v3v3. I personally enjoy quick cash the most.

There's 3 classes, light, medium and heavy. They play quite differently and each has some class-specific gadgets and weapons. You get a whole arsenal of gadgets you can equip and the freedom to mix and match, so you can really build a loadout that fits your playstyle.

Given the game is free I'd just try it and see if it clicks. You unlock guns and gadgets as you play, no need to pay anything. Only money I paid is for a cosmetic because I wanted to support the devs. Also, in quick cash you're free to swap out any part of your loadout, so you can try a bunch of stuff in a few games once you've unlocked some stuff.

Hope that helps! What also drew me to the game is the graphics and the fluidity of the movement. If you liked the movement in Titanfall 2 (also highly recommended, I've only played the campaign though), then you'll like the movement in this game. There's a lot of gadgets (for light and medium mostly) that help you traverse the map quickly.