Energy Transfer
Energy transfer is when energy moves from one object to another, like a moving ball passing its energy to a still one.
Think of it this way
A hot pan sits on a cold burner. The heat moves from the pan into the burner below it. You can feel the burner warm up as the pan cools down. Energy left one object and entered the other.
Explaining energy transfer by grade level
A rolling ball hits a still ball. The first ball slows down. The second ball starts to move. The energy moved from one ball to the other.