The Mixpanel library set to "web" refers to the Mixpanel JavaScript library that is used for tracking user interactions on a web application. This is indicated by the mp_lib property in the event data sent to Mixpanel. When mp_lib is set to "web", it means that the event was sent by the Mixpanel JavaScript library from a web application.
For example, in the table of default properties collected by Mixpanel, the mp_lib property is listed with possible values of web, android, iphone / swift, and unity. These values represent the Mixpanel library that sent the event. So, if mp_lib is set to web, it means the event was sent from a web application using the Mixpanel JavaScript library.
Source: Mixpanel Documentation