An iframe is an object that allows you to embed external content in your HTML page. You can use it to display other web pages, documents (e.g. PDF) etc (although for complex media types you may want to try the object tag instead).
You can add an iframe to your page like so:
<iframe src ="externalContent.html" width="400" height="300">
<p>Your browser does not support iframes.</p>
The p tag inside will display if iframes are not supported by the browser being used.