A longing is a strong feeling of need or desire for someone or something. If you're stuck in detention and all of your friends are outside enjoying a sunny day, you might be looking out the window with longing.
A longing describes an unfulfilled desire. The days might feel long as long as you are longing to see someone you love, if that person is far away. You might be longing for your mother's home-made chicken soup, if you are sick and far from home. Or perhaps you are a pregnant woman, wide awake and longing for pickles and ice-cream at four in the morning. Let's hope there's a twenty-four hour deli open somewhere to help satisfy your longing.