This quest is a little ambiguous but structures basic mean buildings. Bring out the inner arsonist in you and light a fire!

You can use several tools to set a building on fire. Let's go over some of the easiest.

Fireflies. You can capture fireflies in woodland or water areas. These are great for setting structures on fire because fireflies will try and find structures to burn after you throw the jar.

Gas cans. Gas cans are easy to come by in the game and explode when shot setting nearly structures on fire. If you find one laying beside a building shot it and let the fire burn.

Fire bow. If you can craft a fire bow or are lucky enough to come across one you can shot some fire arrows into building to burn them down.

I hope this helps you get this quest.