The skill is called ‘Lone Wolf’ and it enables you to play the game without any sort of help from your partner.
During the game, you’ll always be partnered with someone who will open doors for you, and more importantly help you out of sticky situations. With the Lone Wolf skill unlocked you’ll be the lone ranger. If you get pinned down by a group of zombies, there is no one coming to aid you.
A clearer understanding of the ability is yet to be known. Capcom did confirm that the Lone Wolf is just one of the many bizarre abilities you can unlock in the game.