Bolas are a type of weapon used to incapacitate dragons.
The Hairy Hooligans used them in dragon raids back when they were enemies with dragons. In fact, it was a bola that Hiccup used to shoot down Toothless by use of his bola launcher invention which he called "The Mangler". They used them on dragons again in the Riders of Berk episode, "When Lightning Strikes" when Mildew and a few other Vikings were hunting Toothless when they believed that he was responsible for Thor's anger and making him strike lightning on Berk.
Bolas are also used by Outcasts. In "We Are Family, Part 1", they used them to capture Toothless before another Outcast grabbed Hiccup. They were later used in "We Are Family, Part 2", when Alvin got Hiccup and Toothless to fly at him, while his men threw bolas to capture them.
When Hiccup made his shield, he included a bola launcher in the very center of it. He would later use it many times to incapacitate dragons to give him more time to make his next move.
They are also used by the Berserkers. In "The Night and the Fury", Dagur the Deranged, angered after learning that Hiccup and his father lied to him, used one to entangle Toothless in order to kill him, but Hiccup freed him and defeated Dagur.
When not being used as a weapon, they can be disassembled, and the ropes can be used for other purposes, so bolas have proven to be a handy object to carry.
Bolas are three ropes that meet in the center with small rocks on each end.
Bolas were thrown by the point where the ropes meet. They can wrap around a dragon and sometimes immobilize it. In one case, Hiccup invented "The Mangler", to shoot bolas at dragons during raids. He successfully shot down Toothless and in the crash landing it tore off his tail fin.
The rope on a bola can easily be cut, as has been seen on numerous occasions. In Frozen, the lead speed stinger was easily able to cut through his bonds with his claws and tail. We have also seen Hiccup, with little difficulty, saw through the rope with his dagger in the first film.