V_o / V_in is the expectable duty cycle
(V-in -V_o) is the voltage across the inductor dring ON time
L is the inductance.
T is the cycle time
t_on = cycle_time * duty_cycle = T * (Vo / V_in)
at an inductor: dI = V * t_on / L
So the formula tells how much the current rises during ON time.
And since Vin does not change and V_o does not change ... this also is the current that drops during OFF time.
(true for CCM)