What is the role of activation energy in a chemical reaction

Activation energy is the amount of energy required for the reaction to take place.

This can best be seen in an energy diagram, as the 'hump' that must be reached in order for the reaction to take place. Without the input of the required energy, the reaction will not take place.

The activation energy essentially tells you the amount of energy that you need for the desired reaction to occur.

Yes it would be a physical change.