Reactions happen when particles collide, but most collisions do not lead to a reaction. For a collision to be successful, the particles must hit each other with enough energy to break bonds in the reactants.
The activation energy is the minimum amount of energy that particles must have when they collide for a reaction to happen. It is the energy needed to start breaking bonds.
If colliding particles have less energy than the activation energy, they just bounce off each other and no reaction takes place. If they have at least the activation energy, the collision is successful and the reactants turn into products.
Every reaction has its own activation energy. Reactions with a high activation energy are slow at room temperature, because few collisions have enough energy.