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. For example, in copper, the Fermi surface is spherical, while in graphene, it has a hexagonal shape. Analyzing the Fermi surface provides insights into conductivity, magnetism, and other physical properties of materials.