When things burn, they're on fire. People who live in the country sometimes burn leaves and brush, setting big piles alight until they're reduced to ash.
A house can burn to the ground, and you can burn a stick of incense or a candle with a tiny flame. Another way things burn is from the heat of the sun, as your pale skin burns if you lie on a beach in your bathing suit with no sunblock. Yet another thing that burns is energy, whether it's a motorcycle that burns too much gasoline or a child who burns excess energy by jumping on a trampoline.