When you cremate something, you burn it until only ashes are left. The word cremate is most often used to describe the ceremonial burning of dead bodies.
If you burn a grilled cheese sandwich and there is nothing but black ashes left in the pan, you can say you cremated it. Usually, however, people use the verb cremate when they're talking about a practical or ceremonial burning of a body. Many people are cremated after dying, with their ashy remains either buried or scattered in a beautiful place. The Latin root word is cremare, "to burn or consume by fire."