Peaking is an field monitor function that helps you focus. It finds sharp lines in the image and highlights them with a color (usually red or yellow).
The thicker the superimposed highlighted color is, the more out of focus the object on screen is. This visual aide helps you quickly determine what is in focus, and what is not. Peaking can quickly be engaged and disengaged, to speed up your focus-pulling abilities while shooting in the field.
It can be useful in tricky focusing situations, and when a long shoot is fatiguing the camera operator’s ability to focus by eye alone.