When we talk about the unique visitors to a website, it means the individuals who have visited the website or network at least once within a fixed time frame. It does not count the number of times the visitor might have come to the website, but just counts the identity as 1 unique visitor.
Unique visitors are usually measured on the basis of IP addresses and cookies. Knowing the number of unique visitors is useful for marketers as it helps one understand the behaviors of the users, the customer acquisition costs and could give a bunch of other insights that would help the brand or the page become more effective.