What's the "een" in Halloween?
An HTML tag which tells the browser to display a text entry box on the current page. Any text entered in the box by the user is appended as a URL-encoded query string to the current URL and sent to the server using a GET method.
This is a simple way of making a website searchable or allowing other kinds of simple user input. It relies on the server mapping the query URL to an appropriate process, probably depending on the page in which the ISINDEX appeared. More complex input can be catered for using the FORM tag, or Java.