The internet is a worldwide network that computers can connect to using WiFi (wireless) or a cable (wired). When your computer is connected to the internet, you can visit websites, send emails, and communicate with people all over the world. A strong internet connection is important for these to work smoothly.
The internet is best thought of as a massive web of roads connecting computers all over the world. Just as your house needs a road to connect it to the rest of the city, your computer needs a pathway to connect to this worldwide network. There are two main kinds of pathways: WiFi (wireless) and a wired connection (using an Ethernet cable). WiFi sends information through the air using radio waves, the same way your phone receives radio stations. A wired connection sends information through a physical cable plugged into your computer and into a router. Wired connections are generally faster and more reliable; WiFi is more convenient because you can move around freely.
To connect with WiFi, your computer has a wireless card inside it that can detect nearby WiFi networks. When you click the WiFi icon (usually in the corner of your screen), you see a list of available networks — each one is a different router broadcasting its signal. You pick your home network by name, type the password (sometimes called a network key or WPA key), and your computer is admitted to that network. From there, the router connects you to the internet through your internet service provider (ISP) — the company you pay for internet service.
You can tell how strong your connection is by looking at the WiFi signal icon. More bars or a fuller arc means a stronger signal. A weak signal (one or two bars) means your device is far from the router or there are walls blocking the signal; pages may load slowly or disconnect. The closer you are to the router, the stronger and more reliable your connection. If your signal is weak, moving closer to the router or switching to a wired connection solves most problems.
When your internet isn't working, there's a short checklist to try in order. First, check that WiFi is turned on (there is sometimes a physical switch or a keyboard shortcut that toggles it). Second, check that you're connected to the right network — it's easy to accidentally connect to a neighbor's network or a public network. Third, try restarting your router (unplug it, wait 30 seconds, plug it back in) — this clears up most connection problems. If none of these work, the issue may be with your ISP, which you can verify by checking whether other devices in your home can connect.