If you happen to care, what you were doing with the program Rufus was creating a “bootable media”. Think back in the day when you had to buy a Windows CD and insert that to install or update Windows. This is kind of the evolution of that. An operating system installer can be loaded into a thumb drive (some utilities even let you put many on one drive, and then you can choose between them) and then you tell your computer to read from the USB drive first (which you did via the BIOS boot menu configuration) and instead of booting up your installed Windows, it gives you the option of installing whatever is on your USB drive.
This is fortunately often a pretty painless process, creating the USB boot loaders isn’t hard, and virtually every single Linux distro out there can be installed in this way.
Glad you’re enjoying Mint, and excellent choice for a new Linux user. If you like it, you’ll never need to change to anything else.