It’s a Dota 2 custom map called “Dota Auto Chess” that has very little to do with actual chess.
It’s much closer to a card drafting game. You play against 7 other opponents. Each turn you use money to buy units (from a randomized “hand” drawn from a shared deck), and place those units somewhere on your 8x8 “chess” board. Then, you’re matched randomly with another opponent, and the units automatically fight each other. If you lose, you lose health proportional to number and power of units the opponent had left standing after the fight. Once you get to zero health, you’re out of the game. Last player standing wins.
Each unit has a “species” and a “class”. If you get a certain number of units with the same species or class, you get synergy effects and stat bonuses. You can also combine 3 of the same unit to level it up (and combine 3 level 2 units to create a level 3 unit; thus it takes 9 level 1 units to create a level 3). The synergies and level up mechanics help dictate your overall strategy and what units you should be drafting.
I didn’t get into the economy, interest, win & lose streaks, unit rarity, rerolling, locking, leveling up, neutral enemies, items, positioning, etc, but those are the basics.