An alumnus is a graduate of a school. You can be an alumnus of a high school, college or university.
Want to be an alumnus? Then you'd better study hard and graduate: an alumnus has received a diploma or degree from a school. You've probably heard of some famous or successful alumni (the plural of alumnus from your own school: schools tend to be proud of students who have gone on to do well. Schools also like to ask each alumnus (famous or not) to give back to the school — usually monetarily.