The adjoining figure shows the displacement-time graph
for a body moving with uniform velocity. It covers
distance s1 and s2 at times t1 and t2 respectively.
Slope of line PQ = tan θ = QR \over PR
= s_2- s_1 \over t_2 - t_1 = Displacement \over Time
As Displacement \over Time is velocity, it y, so the slope of the distance-time graph gives velocity of the body.