There are four main types of bridges. The cantilever bridge is one type. It gets its name because it is built using cantilevers – horizontal structures that are supported on only one end. (Think diving board.)
With a cantilever bridge, there is a structure that supports one end of the deck. This deck structure can be built without the need for a lot of supports – falsework – during construction. This is possible because the deck requires no support from above or below.
Once the cantilever is in place, it is essentially a road to nowhere. A second cantilever is built and then a span is used to join the two cantilevers. This creates a deck from shore to shore – a deck that requires no supports other than those provided at the attached end of the cantilever. This simple diagram from Merriam-Webster, shows the basic structure of a cantilever bridge.