Stand-offs hold your board above the (normally) aluminum backing. They are made of brass, non-conductive. If your board was right against the plate, it would short out, possibly kill it. Also keeps your motherboard secure in your case, in event of any movement.
A standoff is used in mechanics and electronics to separate two parts from one another. Standoffs can be many shapes and sizes and made of many different materials. Insulating standoffs may keep two parts from touching each other, thereby preventing an electronic short