You can't - html is pretty "dumb" and just gives the browser some instructions to work with - which is why people use serverside scripting like PHP to create the html for a series of "parts". You'd be hard pressed to find a coder who doesn't work with php, asp, coldfusion, ruby or something similar.
Actually what I think he meant was how to get a page to get displayed in another page. It's pretty simple - using a frame or Iframe as they call them now (you can google it and find some nice articles and tutorials about it). Basically you put something along the lines <iframe src="coolPage.html" width="200" height="200"></iframe> Code (markup): in your html file, where you can set the src to the html document you want and you can control the width and height of the displayed page to whatever you want. Normally I would advise against using frames as it is quite an old standard (though it doesn't stop facebook applications to use it as a way to display content) and now people can do whatnot with CSS and some JavaScript, but if you want to display the contents of one page inside another I can't think up of something better than a frame.
NOT that anyone has ANY business using IFRAME's after 1998... It's been deprecated for a REASON. OBJECT would be the correct tag, but that too has accessibility issues for most instances. Really though we need more details for a proper answer -- are you talking about literally embedding one page inside another, or are you talking about piecing together bits to reduce code and simplify maintenance/updates -- if the latter, you want SHTML, PHP, ASP or some other form of server-side-includes.
Keep in mind that browsers other than IE have a hard time rendering iframes. It's best to merge everything into 1 html document.