Equipment is a game mechanic that changes the functionality of individual cards in the game.
Equipment is worn by the player's Champion, and is divided into 6 categories: Helmets, Armor, Gloves, Boots, Weapons, and Trinkets. A Champion can wear up to 6 pieces of equipment, each placed in the corresponding equipment slot on that Champion, and those individual pieces of equipment will affect cards referencing them in that player's deck. These effects are numerous and can range from changes in how you can build your deck, increasing or changing the powers of cards they effect, and many more.
Equipment can be found from looting treasure chests and completing PvE quests, and possibly other ways. There are well over 1000 pieces of equipment in game currently. Once equipment is found during your PvE adventures it stays in your collection ready to be equipped by your Champion.
An example of a card and it's associated equipment are shown with the card Charge above. In this instance, if you have found either piece of equipment shown during your PvE adventures, and placed it in the appropriate equipment type slot on your Champion, and have included Charge in your deck, all Charge cards in your deck will have the modification text shown applied.
Some pieces of equipment, when worn together by the Champion at the same time, can have a cumulative effect to create what has been called a Set Bonus. These Set Bonuses will provide access to unique, powerful effects, and may bestow unique cards to include in your deck.
Link to the article on Equipment at the official Hex page http://hextcg.com/equipment/