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 understanding how certain properties, called invariants, remain unchanged under group actions. GIT provides tools to classify and analyze geometric structures, such as varieties and schemes, by examining their symmetries. Developed by mathematicians like David Mumford, GIT has applications in various fields, including moduli spaces and representation theory. It helps in constructing quotients of geometric objects by group actions, allowing mathematicians to study complex shapes and their properties in a more manageable way.