Heat transfer refers to how heat energy moves from one object to another one. This might not be easy to picture, but there are simple examples. If you have a bowl of boiling water and you put a metal spoon in it, heat will be transferred into the spoon and the spoon will get hot. Heat is being "transferred" here because it isn't just making the spoon hotter, it is making the water very slightly colder. Heat in this case is moving out of the water and into the spoon. This is heat transfer, and applies to pretty much any scenario where two parts of a system are in contact and at a different temperature. If you try this spoon example out, the spoon itself can get very very hot and is a burn hazard! (Adult supervision required.)