When you reject something, you rule it out or dismiss it. If you're trying to pick a fancy restaurant to go to for your birthday, you'll probably reject the diner you went to just yesterday.
The verb reject has several shades of meaning Reject can mean to refuse to accept or agree with something. If you don't believe space aliens are living on earth, you reject the idea — you just don't believe it. Reject can also mean to turn down something with contempt. If the "cool kids" in your high school reject you, they'll ignore you in the halls and also show contempt by making fun of you behind your back.