Quantum fluctuations are temporary changes in energy that occur in empty space due to the principles of quantum mechanics. These fluctuations allow for the spontaneous creation and annihilation of particle-antiparticle pairs, even in a vacuum. This phenomenon illustrates that empty space is not truly empty but is instead filled with constant activity at the quantum level. These fluctuations play a significant role in various physical processes, including the Casimir effect, where two uncharged plates placed close together experience an attractive force due to changes in vacuum energy. Quantum fluctuations also contribute to the understanding of the early universe and the formation of structures in cosmology.