It is similar to chess/stratego: 2 players move pieces on a grid, taking turns, trying to capture opponent pieces (or reach the opposite corner of the board).
There are 3 types of units: Rock, Paper, Scissors and they can capture enemy's Scissors, Rock, Paper respectively. Each piece can move 1 square in any direction, like a king moves in chess.
I find it very elegant how they use this idea of "different types of units are good against different types of units" in its purest form.
Also, having played ~50 games I find it a very good game, especially now. There are ~30 players in the lobby wanting to play, likely almost all humans, very few knows theory.
The lobby is probably unintuitive/clunky. My method is to turn on Time Control, then click all the "Invite" buttons for other people in the lobby. This allows them to join me, according to my rules. Additionally, if an "Accept Invite (your rules)" appear somewhere, I click on that too.
A single game takes about 6-10 minutes.