Here is an overview of the game, via Nintendo.com:
The free to download online software, Tetris 99, is available as a special offer for Nintendo Switch Online members. In large-scale, 99-player battles, it’ll take speed, skill, and strategy to knock out the competition and become the last player standing. You can target opponents by sending them Garbage Blocks, but be careful…your rivals can target you back! Defeat opponents to acquire KO badges that may give you the advantage on future attacks. Survive the onslaught and look forward to upcoming online events!
View the first screenshots at the gallery.