The Fermi Surface is a concept in solid-state physics that represents the collection of points in momentum space where the energy of electrons is at the Fermi level, which is the highest energy level occupied at absolute zero temperature. It helps to understand the electronic properties of metals and semiconductors, as it defines how electrons behave in these materials. The shape and size of the Fermi Surface can vary depending on the material and its electronic structure. It plays a crucial role in determining various physical properties, such as electrical conductivity and heat capacity, and is closely related to concepts like Brillouin zones and band theory.