The Energy Sector contains companies that are in the business of discovering, processing, or selling (or all 3) natural resources like oil, natural gas, coal, solar and wind.
Oil companies dominate the sector and are the largest players. Energy stocks are also cyclical, meaning that they tend to perform better when demand for energy is high (economic expansions).
Companies in the Energy sector are also very sensitive to changes in the price of the underlying natural resource, like oil. For example, as the price of oil rapidly declined in 2015, falling by 50+%, the earnings for virtually every energy company collapsed.
Oil prices are set by supply and demand but are vulnerable to frequent disruptions and supply controls, set by groups like OPEC.