An "open set" is a concept from topology, a branch of mathematics. In a given space, an open set is a collection of points where, for every point in the set, there exists a surrounding area that also belongs to the set. This means you can move a little in any direction from any point in the set and still remain within the set. Open sets are important because they help define continuity and limits in analysis. For example, in the real number line, an interval like (0, 1) is an open set because it includes all numbers between 0 and 1, but not the endpoints themselves.