The other two poster desribed on what the ESXi host will do to provide the necessary memory - how it will divide the physical will be base on the shares assigned to the VM, if there is a reservation set and if there is a limit set -
Setting a limit will cap the amounto of physical memory the VM can have
Setting a reservation will ensure that the VM will have a certain amount of physical when there is contention
Memeory whares will determin how the memory is divided - for example you have 3 VMs and each has an equal share than will there is contention each VM will get and equal amount of memort assuming reservation and limit are not set - if on VM has twice the shares as the other two VMs than it will 50% of available memory while the other two will split the remaining memort eually pr 25% each