An action of a group on a set is a function (colon-to notation), which is often written (mapsto notation), with omitted from the notation, such that
Equivalently, via Currying, an action of on is a group homomorphism , where is the automorphism group of (so for sets, the group of all bijections , but phrasing the definition this way makes it natural to generalize to other categories). It's a good exercise to verify this; Arbital has a proof.
Group actions are used to make precise the notion of "symmetry" in mathematics.
Let be the Euclidean plane. There's a group acting on called the Euclidean group which consists of all functions preserving distances between two points (or equivalently all isometries). Its elements include translations, rotations about various points, and reflections about various lines.