Help with laptop battery puzzlement?

I've never had a laptop, but I'm borrowing one (Dell Inspiron 5559, if you're curious) - and suddenly had a message taking up the entire screen that the battery is low.

This came out of nowhere, followed about 20-30 minutes later with another that it's "critically" low. No advance warning a few days in advance.

The thing is, I always use it only when it's plugged in, so haven't given the battery a second thought. With the screens that pop up, they're unresponsive. For instance, it will give me an option for "Continue" or "Exit," but when I click, nothing happens. At one point, it switched to a different screen, asking me if I wanted to restore settings, and I had no idea what it meant. When there's a message about losing information, I don't know if it's talking about the Word document on-screen, or the privacy settings, or what?  At one point it asked if I'd seen color bars, and then started flashing solid one-color screens, screeching at me in a scary tone.

I'd appreciate any information. If it's plugged in, why do I need a battery?  Why doesn't it provide more information?