If you add simple and small amounts of HTML code you can check this box to be able to see the result even when you go offline in the editing mode. If you use the portlet to show complete web pages or script you should avoid this option.
Get page from URL
Here you specify the address of the web page you want to insert. This page will be inserted in an Iframe and shown directly on the web page.
Custom HTML code
Here you type you own HTML code that you want to use. Do not type <html> or <head> tags, since they already exist on the web page.
OBSERVE! If you are using dynamic templates you have to set width and height on the HTML portlet in order for it to be visible. This is done by right clicking the portlet and selecting Properties. In the properties you select the settings tab where you can set the width and height in pixels.