Your're seeing a ManagedObjectReference (MoRef for short).
That is a kind of pointer, used in the vSphere API, to another object. In this case that object is a HostHealthStatusSystem object.
You can follow these MoRef pointer by means of the Get-View cmdlet. That returns the object to which the MoRef was pointing.
$esx = Get-VMHost | Select -first 1
Get-View -Id $esx.ExtensionData.ConfigManager.HealthStatusSystem
A MoRef itself consists of two properties, a Type and a Value.
MoRefs are unique and assigned by vSphere. The number 11 in the MoRef you are looking at, is just a unique number, assigned by vSphere when the ESXi node was added..
If you have multiple ESXi nodes, you can list this MoRef for each of them as follows
Get-VMHost |
Select Name,@{N='HealtStatusSystem MoRef';E={$_.ExtensionData.ConfigManager.HealthStatusSystem.Value}}
You'll notice that the number if different for each ESXi node.