A decker is something that has layers or levels, used with a number to show how many. A double-decker bus has two levels of seating, one on top of the other, so passengers can ride upstairs or downstairs. London's famous red buses are double-deckers, giving tourists spectacular views from the upper level.

A triple-decker sandwich has three layers of bread with fillings between them. Some cities have double-decker highways, with one roadway built above another to save space.

You might hear someone describe a double-decker ice cream cone, with two scoops stacked high, or a bunk bed as a double-decker bed. The word emphasizes the stacked, layered quality of whatever you're describing. Whenever you see decker in a word, you can count on there being multiple levels arranged one above the other.