In the night in general we see star used to twinkle or change their brightness but in fact, most of the stars are shining with a steady light. The movement of air in the atmosphere of Earth (with different temperatures and densities) causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to us on the ground. This means that some of the light reaches us directly and some gets bent slightly away. To our eyes, this makes the star seem to twinkle.
In general stars closer to the horizon will appear to twinkle more than others because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky.
