In a broad sense, term traffic conversion means a specific action of a user that is predefined by the site owner or webmaster. Ordering a product or service on the site is the most common example of conversion. Also any action taken by a user on the website can be counted as a conversion. For example, registration on the site, downloading specific content (book, video, archive), subscribing to an email newsletter, staying on the site for more than 5 minutes, viewing more than 10 pages per visit, etc.
Conversion rate is the ratio of the number of people who visited the site to the number of people who made a purchase. Simply put, the conversion rate is the percentage of visitors or users who turned into customers (made an order or another target action on the site). Primarily, webmasters or SEO managers work on increasing conversion, as it is one of the main parameters influencing site revenue.