Attitude
On partner's lead a high card encourages, a low card discourages.
You hold Q 9 3.   Partner leads A from AK.   You play 9 to encourage.

Count
High low shows an even number, usually 2 or 4, low high odd number.

This applies on partner's lead and also when declarer has an obvious long suite:
            K J 10 9 4 3
6 2                               A 7 5
                   Q 8
Playing in NT South leads Q♦.  
West plays 6 (Hi Lo).
East plays low to see what happens next.
When 8 is played West plays 2 and East knows South has no more ♦'s and can take his Ace, holding declarer to 1 trick.

Suit Preference
In a trump contract, it it obvious that you do not want the suit continued then, excluding trumps,
a high card asks for the higher of the other 2 suits and a low card for the lower of 2 suits.

♥'s as trumps.   The Ace is led and there is normally no point in playing the K as it will be ruffed.
                     6
♠ A K 8 2               Q 9 7 2
               J 10 5 3
We need to help partner to switch to the correct suit.   The two remaining suits are ♦'s and ♣'s.
The 9 would ask for a ♦ (high card for higher suit).   The 2 asks for a ♣ (low card for lower suit).

When giving partner a ruff, a low card asks for the lower of the other 2 suits (excluding trumps)
to be led back and a high card asks for the higher.
This means you can tell partner what to lead back to get a second ruff.

♠'s are trumps.
               J 6 5 2
♦ A K 9 4              8
               Q 10 7 3
You play A K and if you then play the 9 you are asking partner to return a ♥  and the 4 asks for a ♣.

to be continued...............