Generally or specifically? In general they tell the search engines about the page. Each tag has a specific function such as page Title shown at top your browser. (This page says "Use of heading tags" if you look at the very top of your browser) They are not relied on by search engines as much as the old days. Now the engines read the whole content looking for relativity. Used to be you could just load the "Keywords" tag and dominate that niche. Now it's better to have content that is relevant than worry about tags. It's still a good thing to learn to use, just not as important these days.
Unless you are talking about H1, H2,..H6 tags in the body section. The original purpose was to add weight to section headings in a post. It is used as a font size by some HTML editors but implies importance. H1 being the highest relevance to the overall article, H2 a sub topic, etc. H1 should closely match the meta tag "Title".
Just2EZ: The original purpose was to add weight to section headings in a post. It is used as a font size by some HTML editors but implies importance. H1 being the highest relevance to the overall article, H2 a sub topic, etc. Right! I've seen some website designers substitute an image of the H1 tag display, instead of using the actual text in the H1 tag to render the page heading. I wonder if they are aware of the (slight) SEO disadvantage that creates for the page. Every little bit helps when it comes to page rank. Hermas
A8ch: substitute an image Good point to bring up. An image won't hurt IF you use Alt tags with a description. The Alt tag in an image does get ranked, and images get ranked. So, using an image properly could actually help the ranking. Without the Alt tag you would definitely lose ranking.
Maybe in near future but still now H1 tags work pretty well and much better than H2, H3 tags at least.