An insulator is a material that blocks the motion of something through the material. So, depending on what kinds of materials properties you are talking about, this can mean a few things.

For example, a heat insulating material doesn't let heat escape through it, like a good winter coat keeps heat in or a cooler keeps heat out (and keeps your drinks cold longer). An electrical insulator keeps electricity from flowing, like rubber, and a sound insulating material keeps sound from passing through.

The opposite of an insulator is a conductor, which is a material that has properties that very much allow for things to pass through. Metals are usually great at conducting both heat and electricity, so we call metals good heat and electrical conductors.