Shares specify the priority of a VM to get resources during a period of contention. When resources in an ESXi host are limited, and the VMs compete to access resources, the VMs configured with higher shares will have higher priority to access more of the host's resources. Shares can be specified as high, normal, or low, with a ratio of 4:2:1, and are applied between siblings in the vSphere hierarchy.
If you do not use a resource pool or vApps, all your VMs will be on the same level in the hierarchy, and thus the shares will be split between all of them, as you can see in the following diagram:

Keep in mind that the shares are only applied during contention. When there is no contention, they are not applied.