The alto flute is a flute that plays a fourth (half an octave) lower than the regular flute. It is longer than the regular flute. Sometimes alto flutes have a bent part at the top end.
The alto flute is a transposing instrument. This makes it easy for someone who plays the flute to play on the alto flute. The fingering for the note C on the flute will give a G on the alto flute.