Ah, this term isn't always agreed on.
But this is my favourite explanation of it so far (not mine):
The phrase in the pocket is used to describe something or someone playing in such a way that the groove is very solid and with a great feel. When a drummer keeps a good metronomic pulse, often referred to as keeping time, and makes the groove feel really good, and maintains this feel for an extended period of time, never wavering, this is often referred to as a deep pocket
Today, the term in the pocket has broadened a bit, suggesting that if two musicians (usually the bass player and the drummer) are feeling the downbeats together, feeling and placing beat one at the exact same time, they are said to be in the pocket.
Whether you are playing ahead (front) of the beat, or behind (back) of the beat, or right on top (middle) of the beat, as long as two musicians (ie. bassist and drummer) feel the downbeat at the same time, theyll be in the pocket.
Many people feel that the question is not so much what the pocket is as much as how you know when youve achieved it. To the musician, it feels like the music is playing itself, as though everything has merged together all the rhythmic parts being played by one instrument.
It goes deeper than that as well. It's the perfect place where everything is locked, and working perfect. It's also about the phrasing and tonality of the notes, but we can go on all day if we get into that!
Hope this is a good start for you...
Technically...what he said. (I do agree). Musically...it's when the rhythm section is playing in "the zone".

I know... I'm a jerk. I actually quit playing in bands, for years, out of frustration.