Hi Diane,
Tags allow you to easily group related topics together for easy retrieval. Use simple words, usually no more than 1-3 words, that describe the tagged item.
So for instance, you post a question related to estimates and you use 'Estimate' as your tag name. Other users have also entered posts related to estimates and used the same tag name. You can then, just click on the tag 'Estimate' and you'll be able to view all posts within that tag. Hope this helps.