EV means Effort value. You can't reduce it but you can increase it. For example, A Noivern needs Sp.Atk and Speed EV. Each EV can reach 252 EV while in total the maximum EV of one mon is 510. so technically you can do 252+252+6 on one mon.
Sometimes an unwanted ev is at max so u need to reset it (costs 10k).
Sometimes, people ask EV trainers to do it for them.
Need me to slide some?