Mock Locations are a hidden developer setting in the Android operating system that allows a device owner to set any GPS location for testing purposes. There are many free apps in the Google Play Store that exploit this hidden setting, using names like "Fake GPS".
The good news is that these apps are very easily detected, and ClockShark is able to report to you anytime that appears to be spoofed using one of these apps. If you receive a notification from ClockShark stating that times for an employee appear to be tampered with using Mock Locations, it means that when one or more of the locations for a segment of time were recorded, the Mock Location provider in Android was turned on.
This means the location recorded is not coming from the GPS receiver on the device. Instead, it came from the Mock Location provider (it is a spoofed location).
Here's a link to an article that discusses "What is Mock Locations" further.