You know when you're walking down the street with some loose change in your jeans (pre-CVD), and with each step most of the pennies and quarters and nickels are swashing around, back and forth with your cadence, sounding like a tambourine every evenly placed step you take, but that one dime way at the bottom corner of your pocket is firmly embedded in place and it ain't going anywhere. In fact you could plot the motion of that dime as you walk and find it always swinging past the lowest part of your step at consistently placed intervals. That dime is in the pocket.
The pocket is where good drummers pitch their tents. It's the center of the rhythmic universe. Time in the pocket adheres to a precise yet flexible schedule. Imagine a traditional clock with rotational parts. The drummer is the dial that regulates the hands. The rest of the band revolves around him. He gently guides their every turn. The collective operation is internally complex yet outwardly effortless. The pocket contains the elusive mysteries of all things metronomic. Seasoned drummers can deploy them on command.