when a person fire a rifle it is advisable to hold the both firmly against the shoulder rather than a little way from it to mimise the impact on the shoulder explain why.

Answer 1

The best first place to try on general questions like this is google.
You will find many examples, illustrations, discussions, and videos.

Answer 2

A rifle has a "kick back" feature that happens when the bullet is fired. A person can be injured by the "kick back" of the gun that pounds into their shoulder or soft tissue when they fire the gun. You put the gun at a spot of great strength to "buffer" the backwards motion of the gun when fired : )
But oobleck's answer is better as you may want the "scientific" explanation, not just the back woods gun owner answer : )