A breaker is a piece of reef

Reef

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

 against which wave

Ocean surface wave

In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...

s break.

This sense of the term is closely related to another sense, that of the breaking wave itself. In times past, when ocean navigation was relatively rudimentary, a line of breaking waves was a crucial indicator that a boat was bearing down upon an island

Island

An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...

 or reef. This was especially the case when sailing in darkness, in which the highly visible line of froth caused by breaking waves could be the only such indicator. In such cases the line of breakers was charted and named as a "breaker".