Those are very big questions. This Wikipedia Page is a good place to start.
The simple answer is, everything humanity does happens in cycles.
But you can think of it as roller-coaster passing through an infinite series of loops. We keep going forward in the long run. But but the repeating loops take us up and down, even upside down and backwards along the way. In every case, coming down each loop gives us the momentum to reach the next one.
Really‽ Okay then