Definition is a statement expressing the essential nature of something (Merriam-Webster).
Definition of something tells you "what that is, what's its essence."
A car can be defined as a wheeled motor vehicle used for transportation, primarily on roads, which usually seats one to eight people, has four tires, and mainly transports people rather than goods.
Explanation is a statement made to clarify something and make it understandable (Dictionary.com).
Explanation is a broader term than definition; it describes in more detail how something works.
Here is an explanation of how a car works: A car uses energy, usually
by combustion of a liquid fuel or from a battery, which is used to
move the wheels, etc, etc.