Nano that generated this files, look at this from GNU Nano website:
In some cases nano will try to dump the buffer into an emergency file. This will happen mainly if nano receives a SIGHUP or SIGTERM or runs out of memory. It will write the buffer into a file named nano.save if the buffer didn’t have a name already, or will add a ".save" suffix to the current filename. If an emergency file with that name already exists in the current directory, it will add ".save" plus a number (e.g. ".save.1") to the current filename in order to make it unique. In multibuffer mode, nano will write all the open buffers to their respective emergency files.
Or in your shell:
$ man nano