Usually this refers to your title tag and meta tags within the head of your (X)HTML.
These should actively reflect the content of your page by containing one or two of the keywords or phrases that appear in the visible content. This helps search engines confirm you are not trying to scam by presenting yourself as something you are not.
The title tag and meta description tag are also displayed in the search results so they should be interesting and attractive enough to encourage internet users to click through and visit your site.
So say you have a site selling bicycle parts and one of your pages is for crash helmets. If your title and meta tags ramble on about "bicycle parts" while your page only mentions crash helmets you are not going to perform well in the rankings.
For full coverage of seo tags visit our page on Meta Tags
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.