The X-Pack is a compact design, with two wings that also act as flaps for changing direction and twin engines set at an angle on the bottom/back. The placememnt of the engines and wings form a rough X shape; hence the name. It attaches to the minifigure via the neck.
The X-Pack is a glider designed for flying relatively short distances at high speed. It's perfect for long or high jumps or traveling between world-islands (not inter-world travel, though). It can be used in battle, as it's small, fast, resiliant and maneuverable.
The X-Pack is an expensive glider, due to its abilities. It can probably be bought once the player has dominated the first tier worlds and half of the second, as you will only have amassed enough cash for it (and still have enough for basic needs) by then. Drake Rock sells these gliders.