The beam of a boat is the measurement of its width at its widest point. Many ship designs have a constant beam measurement that runs most of the length of the vessel, while others may have a more pronounced taper.
Beam measurement is important for two reasons. First, it defines the narrowest channel a boat can fit through, a factor that can be important when avoiding obstacles in shipping lanes. It also has an impact on how fast a vessel may be. Narrower boats tend to be faster than wider boats due to the fact that their hulls have to cut through less water, but a larger beam will be more stable in heavy seas.