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. 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 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.
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.
Bolas are three ropes that meet in the center with small rocks on the 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 the bola tore off his tail fin.
The rope on a bola can easily be cut, as we have 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 some difficulty, saw through the rope with his dagger in the first film.