When you write a new post or reply to a post (but not with Quick Reply) there's a section under the edit box headed "Tags". 

This is a place where you can put some keywords (separated by commas) to indicate what your post is really about - e.g. Avian Flu, Poultry, SFP or whatever.

If enough people tag their posts in this way, it becomes much easier to find related posts.

The relative popularity of the tags is also visualised in a "Tag Cloud" – which appears on the right of the page.

All kinds of things can be tagged - forum posts, blog posts, photographs etc.