What is function of primer?

A primer is a starting point. In my early school days, we passed through primers 1 to 4 in which we picked up the skills of reading, writing, arithmetic, and rudimentary grammar.

A primer in a munition is a small amount of unstable explosive (Mercury fulminate etc) which then initiated the main charge to burn explosively.

A primer in painting is a paint coat designed to prepare the surface to be able to accept a further coat of high quality.