A/The Pocket Rocket is the definite article or slang to refer to a number of memory expansion boards for the Apple II (and possibly other machines).
The name 'Pocket Rocket' comes from the first product bearing the name from Applied Engineering. It was a very small 16kb (by kb I mean kilobyte) memory expansion card for the Apple II bus. The 'Pocket' part is because the card itself is no longer than the card-edge connector and slightly shorter in height.
The Original Pocket Rocket was such a big deal (not so much with the later models) was that the Apple II base model usually came with 48kb of RAM. You could upgrade the memory to the full 64kb (the ceiling in the ][ and the ][+) by purchasing 4 more 4kb memory DIP chips.
However, back in the day, memory was expensive, especially for the high-capacity 4kb chips. For an example, when my father bought our family our Apple ][+, he went for the 16kb internal RAM upgrade for $1200 CDN, or about $80 per kilobyte.
You could make things a bit easier, however, by using of of these Pocket Rocket cards. They were based on the TI 4116 chip and had an interesting feature -- depending on the design, the Pocket Rocket could still use the high-capacity 4kb RAM chips, but it could also use the cheaper 1kb and 2kb chips, though most cards had at most 8 sockets so using 1kb chips would only get you 8kb (and probably wouldn't work).
These cards usually only worked on the 'plus', sometimes on the //e if you were lucky. For the later models you would be better served by the AE RAMFactor or AST SprintDisk.