The evil eye is the name for a sickness or misfortune transmitted by someone who is envious, jealous, or covetous. It is also called the invidious eye and the envious eye.
The evil eye belief is that a person can harm you, your children, your livestock, etc. by staring at them with admiration and/or envy and praising them. But that does not necessarily mean that the person who harbors admiration/jealousy and gives the evil eye is an evil person. Often, it is indeed a genuine admiration and the effect is un-intentional.
One popular way to ward off evil is to wear an amulet called apotropaic charm (from Greek apotropaios, to ward off).
The most common form of apotropaic charm is the evil eye charm, often found on bracelets, necklaces, pendants, etc. which mirrors back and confines the evil eye.