A blunt and/or blunt wrap. A pocket can be a marijuana-filled blunt, or just the wrap (or cigarillo) itself. The name is referenced to the wrap's "pocket-like holding ability". Generally, a common pocket is filled with a gram of delicious dro and smoked amongst a few friends.
The origin of the name pocket refers to the first lines of the song "The Good Times Are Killing Me" by Modest Mouse.
Modest Mouse #1: "Hey Man, can we smoke in here?"
Modest Mouse #2: "Ya, Erik's got those pockets."
"I will be over in twenty minutes with a gram of Sour D and a pocket."
"I'm going to Quik Trip to pick up a pocket"