Parameters for $config_vars, by and large it's an array of arrays, with the subarrays being 2+ items, the first being the type of option (text, large_text, check, int, float), the second being the name of the entry in $txt and $modSettings that it needs to be.
size is used to dictate the size of the box, label is the text label it gets if you don't want to use the default auto populated one, subtext is a sub label it has.
You can simply define it as ('text', 'settingname') to have it saved into $modSettings['settingname'], use $txt['settingname'] and be a basic textbox.
The '' is not outside of the settings array, actually. The array is an array of arrays, really, with one array containing all the settings and each setting being an array or string (default label, not recommended), or '' for divider.