For a reaction to take place there always has to be a certain amount of energy. This energy is needed to be able to break bonds and it is called the Activation energy.
During the reaction new bonds are made and this releases energy.
In most reactions more energy is supplied than released when bonds are made. (so the activation energy is higher than the energy needed to form new bonds) this means that the reaction releases energy to the surrounding and this energy is mostly released in form of heat and therefore it is an Exothermic reaction. (remember EXits are used to go outside so in an EXothermic reaction the energy goes out)
However in some reactions the activation energy is less than the energy needed to form the bonds. So for the reaction to take place energy has to be taken in from the surrounding. This is an Endothermic reaction.
Hope this helps you!