From Archon Arcana - The KeyForge Wiki
Example of the enhanced aember bonus icon in the upper left corner of a card

Each card with the Enhance keyword has added the indicated bonus icons to random cards in your deck. (This has already happened during the deck generation process.) The enhance keyword has no effect during gameplay.

Example: Mutant Cutpurse has “Enhance Damage iconDamage iconDamage icon”. As a result, three Damage icon bonus icons have been randomly added to cards in your deck. The Mutant Cutpurse itself gets no special ability from Enhance during gameplay.

Bonus icons that have been added by Enhance can be identified by the graphical element shown here:

Official rules v1.8 March 2021