A packet is a container or bundle, like the tiny packet of pretzels they give you on an airplane or the packet of papers a teacher takes home to grade over the weekend.
The word packet was first used in the 15th century, from the Middle English pak, or "bundle. Dating back to Britain's Tudor era, packet ships were mail boats that carried bundles of letters overseas. This entire industry of mail service was known as the "packet trade." Today the word packet is more likely to conjure images of things in small paper containers that can be torn open, like the packets of soy sauce you get with your take-out dinner.