Leading low towards your honors 

Definition

'Leading out low' is making the first card you play, from either declarer or dummy's hand the lowest possible card. For example in the spade suit below the declarer leading out low would be the 3 of spades
In the diagram below Declarer is South and dummy is North, in this instance West is 'under dummy' and East is 'over dummy'. As you can see....  if you lead out your high K or Q  from Dummy West will cover it with his Ace when it is his turn to play, and you have no chance of winning a trick with it. You should instead always try to lead a low card from Souths hand and then if West has the Ace he must either play his Ace or the 9 and then you can save the King for when it will win a trick later. If West holds up and does not play his Ace at your first lead of the suit use your entry cards to ensure that you keep leading low from Souths hand this play gives you three tricks if the Ace is under dummy.

The Problem

In Bridge the order of bidding and of play are always clockwise.  In this situation the opponent who is on declarers left hand side (LHS) and before the dummy they have the position described as 'under dummy' if they are the opponent who plays after dummy they are described as as the player 'over dummy'.