Activation Energy
The diagram shows a system with a meatstable state (a), an unstable state (b), and a stable state (c). Energy is required to cause the system to go between these states. 

The "activation energy", DG, for the a -> c process is (Gb - Ga), that for the c -> a process is (Gb - Gc).

If the system is in contact with a thermal reservoir at a temperature T K, the probability of state (a) acquiring the required activation energy from thermal fluctuations in the system is given by: 
P = exp(- DG/kT) , where k is Boltzmann's constant.

From: Barrett, Nix, and Tetelman,
"The Principles of Engineering Materials," Prentice Hall (1973)