You are now in possession of a USB drive that can be used to run/install ubuntu on most computers. Persistence gives you the freedom to save changes, in the form of settings or files etc, during the live session and the changes are available the next time you boot via the usb drive.

Can you save files on a live USB?

A Linux live USB drive is normally a blank slate each time you boot it. You can boot it up, install programs, save files, and change settings.

Is Ubuntu Live persistent?

Ubuntu Live ISOs created after August 1, 2019 support the persistent storage feature, so persistent partitions are only working with Rufus 3.7 and later using Ubuntu 19.10 and later.