FRAND patents have a requirement to license to anyone. Non FRAND patents can be licensed as the patent holder sees fit.
Apple are saying that Samsung are suing them over FRAND patents which Samsung either refuse to discuss licensing, or patents that Apple feel are already licensed, as they are related to hardware produced by a licensee who has already paid a per chip license.
Apple's patent claims against Samsung are supposedly non FRAND patents that Apple would not be forced to license.
FRAND pools come about when implementing a particular standard requires patents from multiple patent holders. To prevent a Mexican standoff between manufacturers who would not be able to produce anything without the other's patents, the patents are pooled, and a set price for licensing the patents is decided upon.
I would have thought that if the patents Apple allegedly infringe are part of a FRAND pool, then this would have stopped Samsung's action a long time ago. Since it hasn't, it probably isn't the whole story, and we'll have to wait to see what the courts say.