A thing that's burnt is one that has been set on fire and burned away. Think of a marshmallow over an open flame - it gets all blackened and charred.
When we say something's burnt, we usually mean it's been damaged or ruined by heat or flames. You might hear someone complain about their food being burnt in the oven, or see a scorch mark on a piece of furniture that got too close to the fire. In some cases, though, "burnt" can also describe things that aren't just physical objects - like when you say your feelings are burnt out from working too hard, meaning they're exhausted and depleted.
(chiefly Commonwealth, UK) simple past and past participle of burn
Example: I burnt the toast.
