An exothermic reaction releases energy to the environment.
This means it makes its container's temperature rise.
The vast majority of reactions encountered will be exothermic.
Remember that all reactions start with bond-breaking - this requires energy and so is endothermic.
However, new bonds will form, releasing energy. And bond formation is always exothermic.
So a reaction can only be exothermic if less energy is required to break bonds than is then released when the new bonds are formed.