Executed by pressing forward and heavy punch, Ryu slides forward and hits the opponent in the stomach with a body blow punch that hits twice. The starting animation before the attack often varies between games.
In Street Fighter Alpha 3 the attack is named Seichuu Nidan Tsuki (正中二段突き, Just Middle Two Stage Thrust) instead a two-hit front elbow lunge to the gut as opposed to a punch but is functionally the same.
In the Street Fighter IV series, it can be used to break Focus Attacks due to it being a double-hitting attack, meaning it can break Super Armor. Also, the attack itself can be canceled to any special after the 2nd hit, and can be cancelled out of a Collarbone Breaker if the input is fast enough.
In past games, however, it's hard to use this close enough to the opponent due to the command overlapping with a punch throw. Though with this in mind, it also works as a solid meaty attack (via wakeup offense) due to its frame advantage and 2 hits.
While the kanji's (鳩尾) literal translation is "pigeon tail", the reading of it is actually meant to be read as either "mizuochi" or "mizoochi", with two other alternate ways of writing it being either 水落ち or 水落.
All three writings and kanji are terms for the solar plexus (the pit of the stomach), or in the case of the latter two examples, literally "the place where water falls".