For many animals light is importantin that they use the diurnal and seasonal variations in light intensity and duration ( photoperiod ) as cues for timing their foraging , reproductive and migratory activities .explain it plz??

This statement means that light is essential for good diurnal rhythm. Diurnal rhythm in simple words mean the sleep-eat-wake cycle of your body. So light determines the time you wake up , go to sleep, etc... it keeps your sync with the day and night constant. Also it keeps in check the seasonal and reproductive changes taking place in your body due to the surroundings. This is exactly what happens in animals. 

Hope it helps!