An invalid timer is a timer that has been deleted or the point configuration for the timer tags has invalid settings.
Validation will list points associated with invalid timers. If a point has no timer associated with it that means the timer has been deleted. If you select the point and click Fix the point will be reassigned to the default timer.
Other timer problems that show up as a result of validation are most likely point configuration errors. The Fix option in the Validation view may or may not be able to fix these problems. However, you can delete the timer points (which is the equivalent to deleting the timer) and re-add it. Any points that were assigned to the deleted timer will be assigned to the new timer as long as the timer names are the same.