Goldstone's Theorem is a fundamental concept in quantum field theory that explains the behavior of systems with spontaneous symmetry breaking. When a system exhibits a continuous symmetry, and that symmetry is broken, the theorem states that there will be massless excitations, known as Goldstone bosons, associated with the broken symmetry. In simpler terms, if a physical system transitions to a state that lacks a certain symmetry, it will produce particles that are massless. These particles are crucial for understanding various phenomena in particle physics and condensed matter physics, as they help describe the low-energy excitations of the system.