Send a file. What is an UUID?

Hey,

I do not really understand how to send a file.
I use curl in php. My Post BODY:

        'files' => [
          [
            'category' => 'cv',
            'uuid' => XXXXX,
            'original_filename' => $_FILES['fileInput']['name']
          ]
        ]

What exactly is an UUID? When I upload the file with an AJAX Call, then I do not have any value that matches.
And why is there no Path to the File?