Conversion rate is the share of visitors who complete a goal, such as signing up or buying, out of everyone who could have. 30 signups from 1,000 visitors is a 3% conversion rate.
It's measured for a specific goal and a specific step: visitor to signup, signup to active user, trial to paid. Each step has its own rate.
Rates vary widely by traffic source. Visitors from a search for your exact category usually convert better than visitors from a general launch feed.
Why it matters for a launch
Raising conversion makes every source of traffic worth more. It's often cheaper to fix a confusing page than to find twice as many visitors.