A pocket is a kind of pouch that's sewn into your clothes. If you are worried about your ink pen leaking in your shirt pocket, then you might spring for a pocket protector. Nice look!
You can keep things like your keys in a pocket, or you can use your pockets to keep your hands warm. Jackets and jeans have pockets, and so do kangaroos and pool tables. In the case of kangaroos, it's a handy pouch in a mother's belly where she can keep a baby kangaroo warm and comfortable. And when you shoot pool, you aim for the pockets. As a verb, pocket can literally mean "put into your pocket," or more figuratively, "steal money." To be "in someone's pocket" is to be under their control or influence.