Chasing every opportunity has a hidden cost: it's the reason you're mediocre at ten things instead of quietly excellent at one.

The thought behind it

What looks free is being paid for somewhere. The sentence points at where, and declines to soften it. In Quiet Power restraint is the whole discipline, and it pays in a currency nobody else can see. Read it again for “one”. That is the point at which observation turns into a decision.