Ace from Ace-King refers to the lead of the ace when holding AKx or longer. Playing standard leads, ambiguity occurs when partner leads a king because he might hold AKx or KQx. Leading the ace from AKx distinguishes these two scenarios.
The ace lead creates a new ambiguity - is it from AKx or Axx? - but this is theoretically less common since players often avoid leading an unsupported ace at trick one.
If playing Ace from Ace-King, you should lead the king from an Ace-King doubleton. (This is the opposite of standard leads, in which the ace is led.) Leading the king followed by the ace tells partner that you have a doubleton (and may be able to ruff the third round of the suit).