Subheadings are one way of narrowing down your search results to a more relevant and manageable set. They're particularly useful if your search topic is very specific. However, if you want to keep your search broad, don't use them. If you do want to use them, keep reading.
Each subject heading has its own set of subheadings, from a set list. Some subject headings have four or five subheadings, some have fifteen or twenty, or more. The number and type of subheadings always depend on the subject heading itself.
You can use subheadings if you are looking at one, two, or a few particular aspects of a particular subject heading, for example you may be interested only in the economics and prevention and control of accidental falls, or the adverse effects, epidemiology and risk factors.
To find out what subheadings you can use for your subject heading, click next to a subject heading to 
select it. You'll see that the appearance of your screen changes slightly, with the addition of a couple of sections on the far right of the screen. One of these sections is 'subheadings'.
To find out what each subheading means, you can click on the page icon to the right of the subheading.
To select a subheading, click in the box next to what you want, to select it. The default setting is all subheadings, but you can choose as few or as many subheadings as you like. Remember the fewer the subheadings selected, the fewer the articles retrieved. Take care not to narrow your search too much.