I like this little metaphor.Most people wouldn't know a pocket if they were gagging on lint. Quite possibly the most overused term in the music world.
"Pocket" refers to the relative positioning between two or more sound sources; be it a drummer and bassist, a kick drum and snare drum and hi hat, a quintet, etc.
When we play as an individual on a drumset, the groove that we create has in its own right a "pocket". It's the way our elements flow together and create motion.
When we play with another musician, it's our relative positioning together and the way we flow that gives the music a feeling of life, bounce and movement. "Pocket".
When we play with a group say, a trio... the relative position of the drummer's kit elements (kick/snare/hats) fitting in with a bassist and guitarist and the margins surrounding each of those elements will dictate a pocket.
When playing music it's as if we are on at least a two dimensional plane, like a football field (measurable in latitude and longitude). Think of the bassist, drummer and guitarist trio as if they are in a sort of a triangular positioning; as long as they flow in that same positioning as we march through the song then they are maintaining some semblance of pocket. If it's an obtuse triangle when the tune starts then it ought to be an obtuse triangle when it's finished and not have morphed into a right or isosceles triangle which is what occurs with bands who are not paying attention to the finer points of time and flow.
Once you can visualize it on this flat field then float the concept of "pocket" into space where you add the "z" axis so you now have Latitude, Longitude and Altitude. (X, Y, and Z axii...)
I also want to paraphrase the late great Jeff Porcaro: Don't worry about terms like "pocket", "behind the beat", and "groove". Just make the music feel good and play what's stylistically appropriate.
If you do that correctly, the other things (like beat placement relative to the pulse) tend to take care of themselves.
Now, if you happen to have such an undeveloped ear that you can't tell the difference between good and bad you need to take it back a notch and actually learn how to listen on a finer level.
Because really, if you can't hear what you're doing wrong, how could you ever correct it?