Activation energy is what kind of chemical reaction that absorbs energy

Activation energy is the energy required by reactants to undergo chemical reaction and given products

Every reactant has some internal energy (sum of all kind of energy like kinetic energy, potential energy, mechanical energy, chemical energy etc). It needs some extra energy to undergo chemical reaction which is activation energy

All kinds of reaction whether exothermic or endothermic needs activation energy

Threshold energy = internal energy + activation energy