I have a health and fitness website and according to moz.com it has: Domain Authority 13/100 and Page Authority 20/100 what does this mean?
Those metrics are owned by MOZ, you can see what they mean in details here and here.. Basically, it refers to the number of backlinks pointing to your domain and how much "authority" it might have. The more authority your site has, the better it will usually rank in search engines.
yeah, it's another way to measure how much powerful is your site anyway, the best measure is how much is visited and how much is useful (conversion rate, which can be applied to many actions, not to selling products only).