This check determines whether all Office Web Apps servers are up to date.
Microsoft releases Office Web Apps updates to ensure that the bugs found in the product are fixed. In addition, new functionality may be introduced with updates. Therefore, it is important to patch Office Web Apps on regular occasions.
Although they are two separate products, make sure that SharePoint and Office Web Apps servers are running on the similar patch level to ensure maximal compatibility.
To verify which build your Office Web Apps servers are running on, start the Windows PowerShell and run the following cmdlet:
(Invoke-WebRequest http://office web apps url/m/met/participant.svc/jsonAnonymous/BroadcastPing).Headers["X-OfficeVersion"]
You can also check the version number in the Version column in Control Panel > Programs > Programs and Features. To see the list of version numbers click here: SharePoint 2013 Build Numbers.
Additional information can be found in the following articles: