What is a wood primer, why its necessary and when should I use it?

A wood primer is a primary coating added to the wood. There are different reasons to use primer, the most important ones are :

1. Primer creates a layer that paint can better adhere to.
2. A primer is required for substantial color changes. If you want to change the color of dark wood to lighter color you need to apply primer.
3. Prior to painting, unfinished wood should always be primed.
   The high-solids content of primer aids in filling in the wood grain and provides a smooth surface for the final coat.