This refers to the general wind direction for a particular region. Wind blows in all directions, but over time, one direction generally “prevails” for a region. A region is largely defined by distance from the equator.
In the United States, we’re used to winds moving east. This is why most weather patterns move from west to east.
However, closer to the equator (e.g. Mexico and Latin America), winds move mostly west.
Why I Looked It Up
A book I was reading discussed trade merchants in the ancient world and how they migrated. It discussed how important prevailing winds were to migratory patterns. I had heard the term, but wanted to understand the strict definition.