Oh no worries! Grub is the boot loader. So you turn on the computer and the firmware looks at its list of things to boot from. When you first installed manjaro it booted from the usb and let you install the OS while running off the usb. Usually you would have had to go into the firmware to adjust the boot priority list, and put usb on the top. Now you can go back in and look at that boot priority list again. Now what you're gonna look for is where manjaro installed grub you can make that the top priority so that next time you reboot it will choose grub, and grub will fire up your linux install. Also in that firmware menu you can look at the other boot options, for example if you have an HP laptop you're going to need to enable legacy booting because the default secure boot settings sometimes cause issues.