If you were inside a train that was moving steadily at the speed of light, would you see your reflection in a mirror? Einstein pondered this question, and it’s implications for the well established concept of relativity. His subsequent theory became known as special relativity (published in 1905).
These slides outline relativity, inertial and non-inertial frames of reference, before introducing Einstein’s special relativity. The consequences of special relativity are also mentioned.