The following diagram shows how a VM gets its resource from ESXi. It is a pretty complex diagram, so let me walk you through it.
The tall rectangular area represents a VM. Say this VM is given 8 GB of virtual RAM. The bottom line represents 0 GB and the top line represents 8 GB. The VM is configured with 8 GB RAM. We call this Provisioned. This is what the Guest OS sees, so if it is running Windows, you will see 8 GB RAM when you log into Windows.
Unlike a physical server, you can configure a Limit and a Reservation. This is done outside the Guest OS, so Windows or Linux does not know. You should minimize the use of Limit and Reservation