At the moment, drugs tend to have a patent of 20 years or so. When you evergreen something, this basically means that you make some slight little change to your formulation. That formulation might not make it any more effective, but that little change would enable you to ask for another patent, and that other patent may be for another 20 years. It basically cuts down any possibility for generic medicines to be in the marketplace.
On April 20th, 2016. See this statement in context.