This is a small amount of computer data sent over a network. Any time you receive data from the Internet, it comes to your computer in the form of many little packets. Each packet contains the address of its origin and destination, and information that connects it to the related packets being sent. The process of sending and receiving packets is known as "packet-switching."
Packets from many different locations can be sent on the same lines and be sorted and directed to different routes by various computers along the way. It works a lot like the post office, except billions of packets are transferred each day, and most packets take less than a few seconds to reach their destination. Even FedEx same-day delivery can't compete with that.