Tags are words which describe a listing.

Sellers add a limited number of tags to their listings. Once they've added an accurate few tags to a listing, interested buyers are more likely to see it.

Smart buyers are watching a set of tags, getting automatic notification of new listings which they might be interested in.

The really smart sellers use the tags with high buyer counts as a good basis for their future creations...

The percentage of listings with this tag that have sold.

And the average sale price is just that - it can give you a sense of how the market values listings with this tag.

In general, it's really useful to keep an eye on these stats, so that you have an idea of what the market thinks of each tag. This way you know you're tagging your listing correctly.