Can someone help me with understanding light relating to astronomy?

This week the teacher brought out a machine that he coud put light bulbs in that were filled with different things (mercury was one). When he turned off all the lights in the room and turned on the machine, we held little squares up to our eyes and could see lines representing the material in the bulbs. He explained to us why different things emit different lights in the spectra (like red or violet) and he also showed us a picture of the sun broken down into distinct bright or dark lines in the spectra. What I don't understand is when we break up the light from distant astronomical objects into spectra, why do we often find distinct bright or dark lines in the spectra? (What produces the lines?) How can we use these lines to find out what an object is made of? If someone can explain this so I can feel confident in my understanding of this... that would be amazing!!