Economist claim that the equilibrium position of each firm in a perfectly competitive industry the equilibrium can be at the point where D=P=MR=AR. At that point, you can notice that the equilibrium market price, Average revenue per unit is exactly equal to the average cost per unit. This means that AR*Q is exactly to AC*Q which means that TR is the same as TC. Since cost and revenue are the same, this means that there is no profit to be gained from doing so?
NB: your explanation should be supposed with a graph for the more details.