So, you know how sometimes you want to install an app, but the Play Store won't let you? Maybe it's not available in your region, or perhaps you want to try out a different version. That's where apk files come in – they're like a shortcut to getting the app you want, without going through the official store.
When you download an apk file, you're essentially getting the app's installation package, which you can then install on your device manually. It's like a little present for your phone or tablet, and it's super easy to do.