Hand signals are a way to communicate and give orders to dragons. It was first introduced in the fifteenth episode of Dragons: Riders of Berk, Twinsanity, by Fishlegs. He orders Meatlug to catch Ruffnut and Tuffnut from a falling branch. Later Hiccup inquires about what he did, and Fishlegs explains to him that he and Meatlug has been working on hand signals, and it seemed to give positive results. Hiccup remarks that it was incredible, and they should start working on it right away, in the Dragon Academy

Hand signals

Here are some of the known hand signals performed on dragons-

Toothless (Night Fury): Plasma blast, Smile, Battle Ready, Search

Meatlug (Gronckle): Hug, Catch

Hookfang (Monstrous Nightmare): Annihilate

Stormfly (Deadly Nadder): Spine shoot, Spin/Turn Around

Barf and Belch (Hideous Zippleback): Gas, Ignite, Come, Go, Eat, Attack