If it is what I think it is, then it's commonly believed that they broke commonly used Diffie-Hellman parameters, allowing them to break any connection encrypted using those.
The parameters can, in theory, be safely used by everyone, and generating them is relatively expensive. But because a few of these parameters were extremely widely used, and they were only 1024 bits strong, it is believed that a gargantuan effort to break them was worth it and the NSA did it.
