https://www.gamespot.com/articles/some-of-hearthstones-boomsday-project-cards-needed/1100-6460766/

Hearthstone's next expansion, The Boomsday Project, is packing a lot of new mechanics into its 135 new cards. Like all new expansions, that means it's going through a lot of play-testing, but one card type in particular needed more attention than most. The powerful spell "projects" were a particularly tricky card type to build and balance.

Projects are spells with symmetrical effects. Instead of just hitting the caster with a positive impact or their opponent with a negative one, it grants both players the same benefit. The idea is to build your deck to make sure you take the best advantage of the project effect, as opposed to your opponent who won't have optimized their deck around it.