A discard system is a system where any card that you follow suit with which will not win the trickA round of 4 cards where each player contributes 1 card. There are 13 tricks in a game of bridge. is known as a discard.

The method or order in which you discard these cards signals to your partner what you like or dislike in a suit (attitude)
This value of the card, whether it is an odd or even card either encourages or discourages that suit.

Basic Premise

To signal which side suit you would like led.

If you play and discard a higher card (usually 6 or greater equals a high card) – the “high-ness” is asking partner to lead the higher side suita suit other than the trump suit in a card game  a long suit held in addition to trumps of the unplayed suits, if partner gets the lead at any time.
On the other hand, discarding a low card is a signal, asking partner if they get a chance to lead they should lead the lower rankedRefers to players bidding a higher ranked suit before rebidding a lower ranked suit side suit of the unplayed suits.

Encourage a suit with odd number discards

A card which is an odd number  for example 9, 7, 5, 3 is a signal that you wish partner to lead the suit of the discard if they have the opportunity. It is an encouraging signal.

Discourage a suit with even number discards

A card which is an even number for example 10, 8, 4, 2 signals that you don’t wish partner to lead that suit if they have the opportunity. It is a discouraging signal.

Examples

The opposition lead out the heart suit, Your partner on their first discard plays a card which shows you how they feel about the suit of the discard and also about a side suit preference:

Partners Discard	Encouraging/ Discouraging suit discarded	Side-suit preferred
5S – a low, odd number	Encouraging Spades	Clubs – the lower unplayed side suit
3D – a low, odd number	Encouraging Diamonds	Clubs – the lower unplayed side suit
7C – a high odd number	Encouraging Clubs	Spades – the higher side suit
4D – a low even number	Discouraging Diamonds	Clubs – the lower unplayed side suit
8S – a high even number	Discouraging Spades	Diamonds – the higher unplayed side suit
2C – a low even number	Discouraging Clubs	Diamonds – the lower unplayed side suit
8D – a high even number	Discouraging Diamonds	Spades – the higher unplayed side suit
9C – a high odd number	Encouraging Club suit	Spades – the higher unplayed side suit
Any card which is a 6 is a neutral card	Not encouraging or discouraging	No suit Preference
Any Card which is an honor cardAn honor card is sometimes referred to as a face cards the Ace (A) the King (K) the Queen (Q) the Jack (J) and the Ten (10 or T) in each suit The 10 and 9 card in any suit are known as Intermediate Cards	Not encouraging or discouraging	No suit Preference