The proverb uses imagery of mice that may be going about their work together. If one can see the mice, one can determine which mice are working and what work they are doing. Since mice are discreet, sometimes one may only see their tails. It is dangerous to count the number of mice at work from the number of tails since some may be dead, unfit or incapable for one reason or another. In this example, our ancestors realized that one should never rely on people who are not equipped to do the task at hand.
The proverb is used to advise on the importance of building capable teams and relying on the genuine expertise and capacity. It can also be used to caution on misplaced trust in a team and politely request for review of the the set-up of the team or task allocation.