Okay kiddo, do you have a phone or a tablet? Good, have you ever noticed that sometimes you can only use them where there is Wi-Fi or else you can't connect to the internet? That's because they need a special thing called a "modem" to help them connect to the internet. A modem is like a messenger who talks between your device and the internet.
Now, a "mobile modem" is similar to a regular modem, but it works with cell phone towers instead of Wi-Fi. This means you can use your phone or tablet to connect to the internet from anywhere, as long as you have a cell signal. Mobile modems are often built into your device, so you don't have to buy one separately. They're what allow you to browse the web, stream videos, and listen to music on your phone, no matter where you are!