What is time?

What is time? That's a difficult question, but this much in advance: Time is not what we think it is. Physics describes time, and also space, contrary to our everyday experience. That makes understanding difficult. We will learn that the properties of space and time are a direct consequence of the constancy of the speed of light. The fact that the speed of light is constant is a statement that cannot even be explained. It is a statement without explanation. A central point of the presentation will therefore investigate the question of what it actually means to explain and to understand.