Freckles are simply small groups of pigment cells (cells that contain color) in the skin. This happens when your some of the melanocytes in your skin over produce melanin, a complex polymer that protects your skin from the sun as well as gives your skin a tan. This is why they are more prominent after sun exposure.
So essentially The less Melanin you have in your skin the higher the chance you have of getting freckles. For some people, freckles can fade and only appear in the summer, while for others they can disappear as they age.