An endothermic change or reaction is one in which heat energy is taken in from the surroundings. The surroundings get colder, so the temperature of the reaction mixture falls.
Examples include thermal decomposition, for example heating calcium carbonate to make calcium oxide and carbon dioxide, and the dissolving of some salts in water: ammonium nitrate dissolves in water and the temperature falls, for example from 22 to 15 degrees Celsius, a fall of 7 degrees Celsius. Photosynthesis is also endothermic because it takes in energy from light.
Everyday uses of endothermic changes include instant cold packs for sports injuries. When the contents of the pack mix, heat energy is taken in and the pack feels cold.