Ok, I don't know if this will work for others, but I thought I would share how I think about the "pocket" when establishing a groove in a song.
Lots of folks talk about having a pocket meaning that you're in sync with the drummer and the rest of the band and have a nice groove going on. That's absolutely true. But it still sometimes seems hard to describe exactly what it is. Here's how I think about it.
Imagine a song starting with a nice strong drum beat, maybe guitars or something but no bass. Lots of songs start this way. If you stick to rock music you can imagine "Fat Bottomed Girls" by Queen or "Shook me All Night Long" by ACDC.
Each time the drum hits the beat - I almost think of it as a machine stamping out the sides of a pocket on a pair of jeans. Thump. Thump. Thump. Almost like a heartbeat on a heart monitor. A series of spikes - each one hitting with the beat. But because there's no bass, there's no bottom to the pocket. There's nothing knitting together the beats from one to the next. If a machine stamped out sides to a pocket on a pair of jeans, but had no bottom to the pocket - you couldn't put anything in it. And that's kind of what those songs are like before the bass kicks in. The "thump thump thump" kind of gets people's feet tapping, but they're not quite grooving yet because there's no bottom to the pocket.
Then when the bass kicks in - it knits together the sides. Creates that foundation between the thump of the drums. Now the "pocket" has a bottom and it can hold a groove.
A deep pocket for me is when the bass is thumping with the drums to make that beat even stronger, and perfectly knits together the beats by creating that bottom. Everything in sync. Not a stitch missing. Nice pocket to hold the groove.
Anyway that's how I think about it.
