On a clear, dark night in your backyard, you can see about 2,000 or so stars in the sky, a small fraction of the 100,000 or so stars that make up our galaxy. They seem to twinkle, or change their brightness. In reality, most of the stars are shining with a steady light. The movement of air (sometimes called turbulence) in the atmosphere of Earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to the ground.
This means that some of the light reaches us directly and some gets bent slightly away. To the human eye, this makes the star seem to twinkle.