When Content Grabber loads a new web page into the web browser, it parses the dynamic web page and generates a equivalent static version of the page. Agent commands will work on the static version of the web page, since it's much faster and more reliable. All action commands will cause a refresh of the static version of the page, since action commands are likely to either load a new web page or modify the existing page.
The Refresh Document command is used to force a refresh of the static version of the web page, so that it corresponds to the dynamic version loaded in the web browser. Usually, Content Grabber will be able to refresh the document when necessary, so you'll only need to use this command in special cases. For example, consider the case where a child web page refreshes the parent web page. After processing the child web page, you may need to perform a Refresh Document command before continuing to process the parent web page.