Phonons are quantized units of vibrational energy that occur in solids. They represent the collective oscillations of atoms within a material, similar to how sound waves travel through air. Phonons play a crucial role in various physical properties of materials, such as thermal conductivity and sound propagation. In the context of quantum mechanics, phonons are treated as particles, allowing scientists to study their behavior and interactions. They can be classified into two main types: acoustic phonons, which are associated with sound waves, and optical phonons, which involve vibrations of atoms in a lattice. Understanding phonons is essential for advancements in solid-state physics and materials science.