An abbreviation for a popular discard method, High Encouraging Low Discouraging. Similar terms are ‘natural discards’ and ‘rubber bridge discards’.
Here is an example:
Suppose you lead the ♥K, losing to the ♥A. On the second round of spades, you can discard a high diamond, the nine, to encourage diamonds, or a low club, the three, to discourage clubs.
Either discard should persuade partner to look for an entry in your hand in diamonds rather than clubs.