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 treat someone with contempt: "My former best friend rejected me to hang out with a different crowd — but I found new friends with whom I had more in common."