The software utility Cron is also known as a cron job is a time-based job scheduler in Unix-like computer operating systems. Users who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.
Bootable USB means that every time your computer starts, it’ll run whatever is in the USB Drive. In this case, it’s your Operating System’s files.
To install (or reinstall) an OS, you need a bootable medium where the files for your OS is stored which will be copied. Most of us will have access to a pen-drive, which makes it an excellent device for booting up OS from.