Each character has standard attack moves where successive hits on an opponent generate a «combo» of several different attacks. (old-games.com)
Because in Mega Man 1, there needs to be a frame between successive hits to count as a new hit. (kobun20.interordi.com)
You generally have to get three points to win the match (each time you eliminate all of your opponents you get one point), but sometimes you'll be on a boss where you first have to eliminate a horde of minions (each of which only require one hit instead of two), and you just have to reduce the boss» health to zero through successive hits. (theswitcheffect.net)