How to schedule a Report in background? what is the use of background job please explain about it?

here are 3 ways to schedule in background:
SM36
SE38
SA38

The easiest of the three is SA38.

Why background? In foreground jobs are only allowed a certain amount of runtime. Long running jobs usually times out in foreground, and have to be run background. Some customers has day-end jobs to fill custom tables, and these only run late at night, so they are scheduled as background jobs as well. There may be any of a hundred reasons why you want a job to run in background instead of foreground, and these are only 2 of them.