Where is the activation complex? IN PICTURE

The potential energy diagram above gives the relative energy of the reactants and the products in a reaction. The x-axis represents the pathway of the reaction, so in this general case, we have:

reactants #->#products

The particles that are the reactants in this reaction have a certain amount of potential energy, which is in this case 100 (y-axis).
We can see that when the reaction is done, the products have a higher potential energy, which is 300.

To start a reaction, the reactants must have a certain amount of energy, otherwise, the products cannot be formed. This is a sort of barrier for the reactants that they first need to overcome before the reactant could proceed. In the diagram, we can find this back as the 'hill' between the reactants and the products. In your picture, this hill has the text "Activated complex".

When the reactants do overcome this barrier, the products are formed. To overcome this barrier the reactants need some energy, which is called the activation energy of a reaction. We can see know, that this activation energy is the same as the height of the hill, and is thus displayed with a.

In other words, if you put in "a" amount of energy (400-300=100) the reaction will proceed and the products will be formed.

For your interest, in reality, this barrier could be an unstable intermediate called the transition state between the reactants and the products. You could look up this page to learn more or watch this video!