The activation energy is the minimum amount of energy necessary absorbed by the reactant molecules to achieve the threshold energy, which is the energy of the transition molecules, after that, the products will be formed.
The reaction only happens if it occurs, so the energy of the molecules must increase until the activating energy. The difference between those energies is called activation energy barrier or transition state, because is when the transition molecules are formed.
In the lake that the rivers lead, water molecules evaporate into the sky and form clouds. In the sky, these water droplets condense and form clouds that will eventually rain.
