In an endothermic reaction, the process is characterized by the absorption of heat from the surroundings. This means that the system takes in thermal energy, which results in a decrease in temperature of the surrounding environment. As the reaction proceeds, the reactants undergo transformation into products, and during this process, external heat is absorbed to provide the necessary energy for the reactions to occur, which may include breaking bonds or forming new ones.
This is in contrast to exothermic reactions, where heat is released into the surroundings, and the environment gains that energy. The fact that the heat is absorbed is significant because it often indicates that the reaction requires an input of energy to proceed, which can impact reaction rates and equilibria. In summary, during an endothermic reaction, the absorption of heat from the surroundings is integral to the reaction mechanism and the changes in energy states of the reactants and products.