MUV stands for Monthly Unique Visitor. A monthly unique visitor is an individual user who visits/accesses your website within a period of 21-days. They could be your potential leads/prospects interested in your product/service. The unique visitors are determined by a cookie that lasts for a period of 1 year or until the users clear their browser cookies for websites.
For mobile apps, the equivalent is monthly active user - an app user who opened your app at least once within the 21-day window. A unique app user is determined as the unique instance of the app installation until the user's app data is cleared or the app is uninstalled.