How is total surplus maximized at the competitive equilibrium?

Question

i mean "why" is total surplus maximised...

Answer 1

It means that at purely competitive markets producer can't set prices and is price-taker. Under such conditions supply curve falls to lowest possible point maximizing total surplus and producers cant' increase their surplus by the cost of consumer surplus.

Answer 2

What do you mean with: "How".

It is maximized at the maximum it could be!