Activation energy simply means the minimum amount of energy needed for the particles to have a successful reaction when they collide. There are two basic factors necessary for a reaction to just take place.
Particles need to collide with each other with minimum amount of energry.They must hit each other in proper orientation. If the particles are colliding each other at proper orientation but not with sufficient energy, then reaction will not take place. Higher activation energy means that more energy will be required for the particles to have a successful collision. Like, combustion. It is generally an exo reaction but still needs heat. It is because activation energy is high. Heat causes particles to gain enough energy to overcome the activation energy barrier. So basically, higher activation energy means slow rate and lower means higher rate