Hello there,

The equal area criterion is a simple graphical method for concluding the transient stability of two-machine systems or a single machine against an infinite bus. This principle does not require the swing equation for the determination of stability conditions, hence the name equal area criterion.

According to equal area criterion  , if the rotor's acceleration area during the fault is equal to its maximum possible deceleration area after the fault is removed, the system is in the transient stability critical state, and reaches its maximum value which represents TTC .

Hope You get It :)