It is an easy way to make an extension cord as good as new at a much lower cost than complete replacement.
Getting the correct wire on the correct connector post is critical to safety, and if you are not sure which is which, then you should not attempt this repair.
Most extension cord repairs (the good ones, anyway) involve a three conductor cable and a three terminal connector.
Your individual wires, after the outside jacket is trimmed back, should be black (or red), white and green.
If you look at the terminals on the replacement connector, one will have a brass (gold) colored screw, one screw will be nickel (silver) and the final one will be green.
Hook them up like this:
Green jacket Green terminal
Black jacket Brass terminal
White jacket Nickel terminal
Assure that no strands of any conductor can touch other terminals and be sure that none of the strands of any conductor are exposed once the conductor cap is installed.