It's a simplification algorithm for a series of points. Briefly, if you have a group of several points relatively close to a straight line between the first and last point in the group, all of the middle points may be removed.

Given a first and last point, it finds the point furthest away from a straight line connection, then recursively divides down the pairs of (first, furthest) and (last, furthest) only if the furthest point is above a minimum threshold distance from a straight line connection.