Conversion rates are the amount of people completing a defined action. Referred to as a conversion, as the user has converted from a visitor to a customer, or a visitor to a lead. You get the idea. The idea is to capture areas where potential customers, change to something else. This helps to give intelligence on how successful an initiative is.
You might analyze conversion rates, by piece of content or URL, traffic source, time of day, device, geography, to find where and what is converting. And to find areas of improvement.
To measure conversion rates, typically an analytics system is used, that helps capture when these events happen and send them to a central dashboard, where you can see the results. These are usually in real time, or as close to real time as possible. So you can see what the conversion rate is at any point in time. And then make adjustments to your marketing activities.