In some desktop application, there are contextual help applications, where you would click on a help icon, and then click the item you want help on.
It is called "contextual" help because the help that is shown is very specific to the item you need help on, as opposed to the general help, which documents the entire page.
As developers, we like to write concise code. So, when asking for an email address, we might simply write this HTML:
<tr><th>Mobile</th><td><input name="mobile" /></td></tr>
However, help text tends to be much larger. Obviously, if you don't understand immediately what is required, then it needs to be explained in-depth.
If we were to embed the help text into the HTML source for every element in the page that we intend ...