Q is the energy that is stored in a pendulum divided by the energy that is dissipated during every period of oscillation:

Its maximization is an important prerequisite for the construction of an accurate clock. In a typical pendulum clock, It can be obtained using a high mass bob (great stored energy) aerodynamically shaped (low air resistance and consequently small lost energy per period of oscillation).
A high value of Q means a reduction of the force used to keep the pendulum in motion. Any slight variation of this force disturbs the oscillation. Reducing the applied force, its control is improved and this increases the clock stability. Furthermore, other energy losses deriving from friction in the escapement motion are minimized.

This picture shows the value of Q for the Littlemore clock, the most accurate mechanical clock all over the world, when air pressure changes.