Basically an open skies agreement is akin to a free trade agreement, where we remove any restrictions between our national carriers to be able to fly into each other's markets, the number of destinations they can fly to, the frequency of flights per week, and the amount of cargo they can carry between the markets. It's basically an open agreement that covers all of the basic air freedoms under an air transportation agreement, with the exception of cabotage, the ability to pick up passengers in a second market and carry them within domestic destinations in that market.
On February 2nd, 2015. See this statement in context.