In bridge your signal tells partner what she needs to know.  When your partner is leading a suit, she needs to know whether to continue the suit either now if she has the lead or later if she gets on lead again.  The attitude signal tells partner whether to continue the suit or not.  When partner is winning the trick or dummy is winning the trick, a low card tells partner to continue the suit.  A high card tells partner to try another suit.  We cannot signal partner when we have to play “third-hand high” to try to win the trick.  This is called “upside-down” attitude and it is superior because you use a low card in suits you like and I high card in suits you don’t.  Some people may have been taught “standard” signals where high says you like it and low says you don’t.  You can play either.  You and your partner have to agree.

low  = likes
high = hate

You are not giving an attitude if opponent leads a suit.

Against both suits and NT, you play a low card when you hold the honor above or below the sequence led.  For example, if partner leads the K promising Q, you would play a low card with either the A or the J in that suit.  Without either of those cards, you would play a high card to tell partner to stop playing the suit.  

When partner leads a low card and declarer wins the trick in dummy, you play a low card to tell partner to continue the suit if you have either of the next two lower cards below the one that dummy played.  For example, if partner leads a spade and declarer wins the A in dummy, you would play a low card if you have either the K or Q of spades.  

Against suit contracts, if partner leads the A promising the K, you play a low card when you have a doubleton because you want partner to continue the suit by playing the K and then playing another so that you can trump.

When partner leads the A from AK, if you hold the QJx or QJxx, you play the Q.  This is a very special signal that tells partner that you have the J and that she can lead a low card to your J if she needs to put you on lead.

When partner leads the K from KQ, if you hold the J10x or J10xx, you play the J.  This is a very special signal that tells partner that you have the 10 and that she can lead a low card to your 10 if she needs to put you on lead.