When you boot your pc to that usb stick (bootable flash drive/thumb drive).....it will be running in live mode. (it actually runs in the pc's ram) If you pull the usb stick out the pc will crash/stop. If you attempt to save some info, and then reboot the pc and start again using the same usb stick....the info will not be saved .
However....if you boot your pc to that same usb stick, and then click on the "install linux mint" icon on the desktop.....Linux Mint will be installed to the pc's Hard Drive....... Permanently.......and there is no need to plug the usb stick in again. At the end of the install you can pull the usb stick out and reboot the pc in the normal way
Be aware that installing it like this will wipe whatever is on the hard drive...so be sure you have saved anything from that hard drive that you want...preferably to an external hard drive or to another usb stick.....depending on the amount of stuff you need to save.
I deliberately made this answer long....so you would be made fully aware. Read carefully.