The Spyglass is a handheld device, similar to a telescope, seen throughout the Dragons TV series.
Spyglasses are used to see far off things up close so they can be observed in greater detail. Tuffnut also likes to use it to see things as if they were further away. It has been used on multiple occasions by Hiccup, and other characters. It is possible that Hiccup invented the Spyglass, as he is the only one to use it whilst still living on Berk during Dragons: Riders of Berk and Dragons: Defenders of Berk.
Similar to the appearance of a telescope, it is probably made of animal skin, wrapped in a conical shape, with lenses on each side. It is fairly small, as it can be held in the hand with ease, and stored in a satchel easily.
Spyglasses are seen throughout the Franchise, including, but not limited to, the following appearances.
A spyglass is first seen in the episode "Dragon Flower" when Hiccup offers one he made to Trader Johann in trade for a jar of squid ink. Johann refuses the trade however, reminding Hiccup that he's already traded five just like it, yet he still only has two eyes.
Mulch and Bucket are seen using a spyglass at the beginning of episode "Have Dragon Will Travel, Part 1". They are watching an unknown Dragon Rider on a Razorwhip attacking a ship. Later in the same episode, Hiccup uses one to spy on Heather's activities.
Hiccup uses a spyglass in the episode "Defenders of the Wing, Part 1", when inspecting the island called Caldera Cay. The Dragon Riders see a strange village and a large statue of a dragon species they hadn't seen before. It is later revealed to be an Eruptodon.