Visualforce is a framework that allows developers to build sophisticated, custom user interfaces that can be hosted natively on the Lightning platform. The Visualforce framework includes a tag-based markup language, similar to HTML, and a set of server-side “standard controllers” that make basic database operations, such as queries and saves, very simple to perform.

To know more, check the below link

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_intro_what_is_it.htm