From Zelda Legends Wiki
Deku Nut is shaped like an original nut, but there's light coming from inside the nut. By throwing Deku Nuts, it's possible to stun enemies, and other things. When Deku Nut hits the ground (or anything hard), it will explode and create a group of yellow, shiny smoke.
It's possibly a rumor, but it's possible that the thing the Sheikah throw down when they are about to leave/disappear, is a Deku Nut.