The proportion who act.
WHAT IT IS
Visitors who did what you wanted, as a proportion of all visitors.
WHY IT MATTERS MORE THAN TRAFFIC
Doubling your conversion rate has the same effect as doubling your traffic, and is usually cheaper.
WHAT A LOW RATE MEANS
Visitors arriving are not the right visitors, or the site is not persuading them.
Those need different responses.
HOW TO TELL WHICH
Look at conversion by traffic source.
If one source converts and another does not, the traffic differs.
If none convert, the site is the problem.
WHAT AFFECTS CONVERSION
Whether the page answers their question Whether contacting you is easy Whether they trust the site Whether it works on their phone Speed
WHAT TO IMPROVE FIRST
The pages receiving the most traffic with the lowest conversion.
That is where the largest gain sits.
WHAT NOT TO COMPARE
Your rate against published averages. They vary enormously.
Compare against your own previous figures.