Watch the What Is Groove? online guitar lesson by Pat Martino from The Nature of Guitar
When musicians refer to a state as a "groove," it's the same thing as a person who is having dinner and says "wow..that was incredible." A groove is when it's really happening.
The same song is different every time, depending on the groove. Sometimes it has a groove and sometimes it doesn't. The same thing with the food; sometimes it's exquisite, sometimes it's terrible. The word groove is the ecstasy of something that fulfills you.
At this stage, life has become a groove for me - I can accept all of what is pleasurable or not. Both are part and parcel to polarity.