Basically you're modifying a string that consists of:

   IP Address:Port:Virtual Host

For example:

   172.16.1.1:80:www.example.com

The raw values reside in an XML config file called "applicationHost.config" which replaces the old school and rather opaque IIS6 metabase.

Admittedly that example is a bit old school, there's better commandlets available now that wrap having to manipulate these string values directly. e.g. the New-WebBinding and Set-WebBinding examples shown in sibling comments in this sub-thread.

A site config looks like:

  <site name="MyWebSite" id="3" serverAutoStart="true">
    <application path="/" applicationPool="MyWebSite">
       <virtualDirectory path="/" physicalPath="D:\websites\mywebsite\www" />
    </application>
    <bindings>
      <binding protocol="http" bindingInformation="172.16.1.1:80:example.com" />
      <binding protocol="http" bindingInformation="172.16.1.1:80:www.example.com" />
      <binding protocol="https" bindingInformation="172.16.1.:443:" />
    </bindings>
   </site>

None of this is particularly mysterious, and the documentation is pretty good.