Geometric Invariant Theory (GIT) is a branch of mathematics that studies the action of groups on geometric objects, particularly in algebraic geometry. It focuses on classifying geometric objects, such as varieties, by examining their invariants under group actions. This theory helps in understanding how these objects can be transformed while preserving certain properties. GIT was developed by mathematicians like David Mumford in the 1960s. It provides tools to analyze and construct moduli spaces, which are spaces that parametrize families of geometric objects. By using GIT, mathematicians can simplify complex problems in geometry and algebra by focusing on invariant properties.