It's a deceptively simple explanation....but still a tough one to put in words, for me at least. As usual I'll make up for that by using MANY words!!
I can tell you it has nothing to do with application of higher technical ability (in a shredding or hot-licks sense, at least) ....you don't need to be a virtuoso to be 'in the pocket.'
I'd suggest that you do however have great technique. Then again, I've seen drummers who 'look' uncomfortable, as if they're NOT in the pocket -but are awesome to listen to. Cindy Blackwell with Lenny Kravitz is just one example IMO....to me it looks like she's trying and working way too hard to be in the pocket....but then I listen to her and feel she's dead on the money much of the time. Just my opinion tough.
It's pretty simple in the end...the 'pocket' is just playing in such a way that makes everyone feel really @$%# good about the music. It's serving up the grooves that really compliment the other players and the song.
Usually this happens in a simple fashion....not too flashy or overly technical playing. Not to say that it can't be busy though. I'm sure many people consider Carter Beauford to be 'in the pocket.' Steve Gadd on Chick Corea's 'Sicily' was in the pocket IMO with those busy inverted paradiddles.....and 50 Ways To Leave Your Lover' to me was somewhat busy but still in the pocket. There's some level of subjectivity involved in the assignment of "he's/she's in the pocket." Some say Starr or Watts are always in the pocket....others will say never.
Funk and R&B players were likely the first to use the term....more and more you'll hear it used regarding players of rock, reggae, jazz and other styles as well. I don't think it's a style specific thing so much anymore.
Most important to us drummers though is attaining that fat 'double pocket' ....that's what you should really strive for....or playing in the 'double breasted pouch' as it's known in Australia. OK, yeah....I'm tired and just making stuff up now.....sorry.
