ISO files are disk images often used to distribute software. In years past, we burned them to CDs. As the ISOs themselves became larger, we’d burn them to DVDs instead. In either case, we would then boot from the CD or DVD to run whatever the software provided. A good example might be operating system installation DVDs.
More and more machines are coming without optical drives — that is, they don’t have the ability to read a CD or DVD, much less boot from it.
Fortunately, there are tools we can use to take an ISO that contains a bootable image and place it on a USB thumb drive from which you can boot.