$_FILES is a superglobal, much like $_GET or $_POST, that stores the information about the uploaded file.
If the name of your upload input form element is 'myfile' then, after uploading you will have an assiciative array $_FILES['myfile'] storing all the data about the uploaded file. To learn about it, just var_dump() it:
var_dump($_FILES['myfile']);
All the uploaded files are usually first uploaded to the /tmp directory (or whatever directory is set for this purpose in the php.ini file). After doing some sanity checks about the file (like testing for the right mime type), you will want to move the file to it's final destination. That's what the move_uploaded_file() function is intendedn for.
In this case $_FILES['userfile']['tmp_name'] is the temporary file path of the uploaded file and $uploadfile should store the final path of the file.