We all know what time is — we experience it every moment of our conscious lives. If there is one thing that we all have in common, it may just be the passage of time as we get older.
But once you actually start trying to provide a definition of time, it eludes our grasp as if it was so much mist. Trying to come up with an explanation without using self-referential terms seems really difficult.
Maybe someone else will have better luck than me: What is time?
Related questions: What does “now” mean? Is free will important? What things do all people have in common?