In DDR and ITG, Hands refer to a row of arrows in 4-panel gameplay that have more than two notes. They are named as hands because the players must slam their hand/hands on the panel in addition to stepping or holding freeze arrows with their foot.
They are typically seen on ITG charts, which is a way to increase the difficulty in a song.
While they are unheard of in DDR, a song in Dancing Stage 1.5, Uh La La La by Alexia, one instance of hands is seen wherein a hand plant is placed (possibly mistaken) in the middle of a jump stream. There are also a handful of charts in Solo (6-panel mode) which technically can qualify as hands, but are still reachable using only both feet.