Simple HTML in this case means "without the HTML 5 extras".
Probably it also means without the things from earlier HTML that are
deprecated, or just fancy. I concentrate on what I find useful, and
I try to express things with as little 'weight' as possible.
If you didn't see it first, you may need to read
the introduction first
(especially to understand why a page about a text mark-up language
only seems to talk about tags).
content makes it understandable.
You may think that it's only got to look structured,
and that will do - you may be right:
if no one ever needs to do anything other than look at it.
But you probably really want a search engine to be able to
display the most relevant bit of your page;
you want someone with visual difficulties to have it read to them,
or be able to do things to make it highly readable.
After all, what you're trying to say is most important to you, isn't it?
Here's an example of how part of this looks without structure - is it improved?
Would you really like to read a page like this?
Content Following the h1, we're ready for content:
Paragraphs - <p> tags Lists - <ul>, <ol> and <li> tags
Quotes - <blockquote> Pictures - images, video, animations, charts
Tables - for data
Every piece of content should have a single title,
and that should be the h1 in that content.
If the piece of content is a web page,
it should also be the title of the page (in the page's header).
And, after an h1 we can have as many h2s as we need to
introduce sub-sections of our content.
So the h2s are also followed by content, and, maybe, even h3s.
And you're allowed h4s inside h3 content, and h5s inside h4 content, and so on.
The result is the same as (and can be achieved by) outlining.
In fact, there's an excellent on-line outline checker -
you should test web pages (including this one)
Better still, in FireFox, install the WAVE toolbar,
and don't just test outlines, but accessibility generally!
You should also check pages against Validators to ensure that
the HTML you've written is actually correct -
don't be disheartened, these tools exist to be picky -
remember that anything on a website must survive in all
sorts of different web browsers; anything that they don't
understand could mean that all your work doesn't achieve the
desired result in some odd-ball case.