Heat is responsible for all types of energy transfer other than those that fall under the definition of work. One easy example to think about is ice cream melting on a hot day. Because the ice cream is at a lower temperature than its surroundings, heat flows from the surroundings to the ice cream, causing its temperature to increase, and eventually it melts. There are lots of other examples of heat flow too; it’s a pretty big category since it covers all types of energy transfer that aren’t defined as work.