And probably you've noticed that the traffic size is measured in packets. What are those packets after all, and what do they serve?
A packet is (as the word goes) a packet of data transferred through the internet. Trillions of packets are transferred through the internet every second. Suppose that you try to visit a website. When you select the URL you want to go, a packet of data departures from your PC. This packet will finally arrive at the server in which the specific website is hosted. The server will then answer with a set of packages that will probably follow different roads, but still they will all arrive at your PC. When these packages are "joined" again in your browser, you will eventually see this website!
You may now wonder, why does a server fragments the site into many smaller packages instead of sending it into one big package? This video explains the reason in a simple and understandable way, using the all-day road traffic.