As multiple virtual machines run on the same ESXi host simultaneously, vSphere has a lot of mechanisms for proper resource management.
To start with resource management, it is sufficient to understand the concept of resource controls. There are three configurable parameters to control virtual machine's access to a given resource. These are:
Shares: Shares specify the relative priority or importance of a virtual machine's access to a given resource in case of resource contention.
Limits: This is the maximum amount of CPU or memory that can be consumed by a virtual machine.
Reservation: This is the minimum amount of CPU or memory that is guaranteed to a particular virtual machine. The virtual machine will not power on if its reservations are not met, to avoid poor performance.