HTML headings are sections of your web page, and range from H1 (most priority) to H6 (least priority).
In general, the H1 is used for the title of the individual page (for MereChurch sites, your page title will be an H1).
Think of headings as creating an outline of the contents of your page. As such, each heading should be descriptive of the content below it, and each heading should make sense within its parent heading.
Use H2 headings to mark major divisions of the page content, and then use H3 headings to mark subdivisions within the major divisions. H4, H5, and H6 elements should be used sparingly, but can be used to further subdivide the page.
Good:
h1. Fruit
h2. Apples
h2. Oranges
h2. Bananas
Bad:
h1. Fruit
h2. Hammers
h2. Oranges
h2. Bananas
Also Bad:
h1. Fruit
h4. Apples
h2. Oranges
h3. Here's an unrelated heading
In general, each of your headings should be unique, and they should make sense within the flow of the page. When you put together your page as a cohesive whole, rather than adding a bunch of unrelated topics, you are serving your website visitors and helping them to find what they are looking for more quickly!