When you reject something, you decide NOT to take it. If you're shopping for eggs and you see a carton with a broken egg, you should reject that carton: put it back, and don't buy it. You don't want that one!
If you tell your friends, "Let's build a fort!" but they say "No, that's stupid; we're not doing that," then they've rejected your idea. If you try out for a play but you don't get in, you've been rejected. If a high school student really wants to go to a certain college, but the college sends them a letter saying "Sorry, no, you can't come here," then the college has rejected that student.
If you walk up to a group of kids playing basketball and say "I'd like to play, too!" but they say "No, you can't play with us," then they've rejected you. Rejection can really hurt your feelings.