All organisms have an inner «clock» that follows the rhythmic change of day and night, light and darkness. It is not one single organ, but rather a complicated interplay between organs across mainly our nervous- and hormone systems. Sun light is the most important signal for biological clocks.
Biological clocks tell the organism which time of day it is (see also circadian rhythm), which season it is and where in its life cycle the organism is (see for example the life cycle of freshwater mussles). This is critical information for the organism to determine when it is time to eat, rest, breed, gain weight, hibernate, change fur and so on.