Wave Function Collapse is a concept from quantum mechanics that describes how a quantum system transitions from multiple possible states to a single state when measured. Before measurement, particles exist in a superposition, meaning they can be in multiple states at once. When an observation is made, the wave function collapses, and the system assumes one definite state. This phenomenon is crucial for understanding the behavior of particles at the quantum level, influencing theories about the nature of reality. It is often illustrated using thought experiments like Schrödinger's Cat, which highlights the paradoxes of quantum superposition and measurement.