What does vms in problem actually mean?

On the "VMware vCenter / Virtual Datacenters Summary" report what is used to determine if a vm is "in problem"?

There seems to be quite a few "vms in problem" in multiple VDC's but I'm unable to work out what is wrong, which specific ones are "in problem" or why they are reporting this way. These numbers do not seem to have changed in quite some time either even though the report shows that it is only reporting for the last "1 day".

Any ideas on what this column actually refers to would be helpful.

Thanks in advance

Jason