The result is a String, which is the name of a file that doesn't exist. It can be used for a temporary file. The name is different for at least 26 consecutive calls. Example:

:let tmpfile = tempname()
:exe "redir > " .. tmpfile

For Unix, the file will be in a private directory tempfile. For MS-Windows forward slashes are used when the 'shellslash' option is set, or when 'shellcmdflag' starts with '-' and 'shell' does not contain powershell or pwsh.