Some more info on what exactly the BIOS is, since you don’t usually run into it while using Windows: It’s a tiny, low-level program that comes with your computer’s motherboard that controls all the fundamental stuff about your system. It can enable/disable wifi, USB ports, CD drives, etc.; set your system’s time; allow or disable weird stuff like CPU overclocking or Virtual Machine support; change power management settings (like whether to use the computer’s battery); and importantly, decide which operating system to use. Your computer actually always goes through the BIOS before starting Windows, it just won’t show you any of these settings unless you ask it to. That’s why you need to go to the BIOS after installing Linux, you need to tell it to use Linux from the USB stick instead of Windows like it usually does. The icon on Linux Mint that installs it to your computer just copies Linux from the USB to your computer and tells the BIOS to boot into Linux instead of Windows from now on. This is also what lets you “dual-boot”: you can have both Windows and Linux on your computer, and the BIOS chooses which use, or you can ask it to switch to the other one.