"The end Justifies the means" is a summation of a school of thought called consequentialism. Simply put, it means that an action should be judged by results, rather than any characteristic of the action itself. Knocking someone down is justified if someone else is shooting at them.
It means that the outcome or result is worth whatever is done to accomplish it. Often the way of achieving this goal would in normal circumstances be undesirable or possibly even immoral. For example, eating human flesh to stay alive in a survival situation.