They explain what the variables mean in the function header block;

   * @param {Window}        i The global context object.
   * @param {HTMLDocument}  s The DOM document object.
   * @param {string}        o Must be 'script'.
   * @param {string}        g Protocol relative URL of the analytics.js script.
   * @param {string}        r Global name of analytics object. Defaults to 'ga'.
   * @param {HTMLElement}   a Async script tag.
   * @param {HTMLElement}   m First script tag in document.

It might be easier to read the code if they used different names, but any good refactoring tool will replace them with your own names without any problems.