Conversion rate is conversions divided by visits. If 2,000 visits bring 40 enquiries, the conversion rate is 2%. You decide what counts as a conversion based on what the site is for.
Why it is the number that matters most
Doubling traffic is expensive. Lifting conversion rate from 1% to 2% gives the same result, and a clearer offer, a shorter form or a faster page is often enough.
What to compare it with
Averages from the internet say little, they vary by industry, price and traffic source. The best comparison is your own conversion rate before and after a change.

