Coefficient of friction (µ):

            The ratio between the force of limiting friction F_s and the normal reaction R is constant. This constant is called the coefficient of friction and is represented by µ

Thus    $\mu=\frac{Fs}{R}$ ......(i)

Or   $Fs = \mu\:R$ ......(ii)

If m be the mass of the block, then for horizontal surface,

                      R = mg .......(iii)

Hence $Fs = \mu\:mg$ ........(iv)