"No one knowingly does evil/wrong"
Socrates says this because the root of human instinct is to obtain happiness or to be happy. Therefor people may do things that they believe will bring them happiness but in truth is a bad thing and actually causes them pain, sadness, or regret.
Heres an example: You see a guy walking down the street and he drops his wallet. You pick it up and take a look inside and see theres $500 in there, you decide to take the money and leave the wallet on the ground.
By taking the money you think it will bring you happiness so you take it not thinking that its really wrong and if there are any other consequences that may come from taking it.
The idea being that you do wrong because you do it out of ignorance of the consequences of your actions. Socrates believed this principle so strongly that he attributed knowledge with being good, because the more you know the less ignorant you are so the less reason to do wrong/evil.