A batsu, drawn as an "X", is the
Japanese symbol for "wrong" or "incorrect".
This is in opposition to
maru, drawn as "O", which is used to connote roundness and correctness. A
sankaku, or triangle-mark, is used as a sign of partial completion.
Sometimes, people play a
batsu game--in which if someone fails a dare or loses at
jan ken pon, that loser must suffer some penalty or do some embarassing act/favor.