Major and Minor are terms used in the Solunar Theory to describe the best (major) and second best (minor) bite times each day.
In other words both are good times to go fishing.

The minor times are usually a bit shorter and less vigorous. 
But depending on your location, the weather, sunrise, etc they might be just as good or even better on a particular day.
Give both a try and see which work best for your fishing spot.