A meteor is a sand- to boulder-sized particle of debris in space which can cause a lot of damage when it hits something. Technically it should be referred to as a meteorite, whereas its path as it enters an atmosphere is called a meteor, and after it has made impact it is called a meteorite. But we're just going with the way people actually talk, okay?
Meteors usually burn up in a planet's atmosphere, unless they are large enough not to. Lots of meteors falling together or in rapid succession are known as a meteor shower.
Even meteors that remain intact seldom pose a threat to anyone who isn't standing directly beneath them, although they can introduce alien bacteria or other invasive species, such as S.L.I.M.E. An organization called the Meteor Police is dedicated to apprehending any and all dangerous or criminal meteors.