There Isnt really a goal to the game except to kill the enemy king, and its mostly supposted to be a fun little minigame when youre bored
I will add deselecting by clicking off in the megachessatron update after the jam, and possibly a chesshandbook in the game to tell the differences between pieces
for now:
-king: the most defensive piece in the game, also pretty decent for attacking
-queen: a stronger bishop with less recoil
-bishop: a suicide bomber, second highest speed in the game but really light and takes a lot of recoil
-rook: second heaviest piece in the game, also has pretty decent attack when used at close range
-knight: jumps up and can bounce on pieces, great for clearing areas of lighter pieces for attacking pieces to go through
-pawn: Almost does nothing, but its angle of attack is great for knocking pieces over
Hope this answers your questions!