Entrances are defined as the first pageview of a session. If Bob types in your website address and comes to your homepage first, that first pageview is also the entrance and the homepage gets credit as the entrance page.
How Are Entrances Different from Sessions?
An entrance is recorded on the first pageview or screenview hit of a session. In contrast, Sessions is incremented on the first hit of a session, regardless of hit type (events, for example, could create a session but not an entrance). Thus, there may be a discrepancy between Entrances and Pageviews or Screenviews.