Meaning of a finite mean-reverting level

I am studying Quantitative Analysis, AR model.
It says “all covariance stationary time series have a finite mean-reverting level”.

I don’t quite understand this sentence. I know mean-reverting concept. If the outcome is greater than mean, it will come down to mean in the next period, if the outcome is less than the mean, it will come up to the mean. But what does it mean by the finite mean-reverting? Is there infinite mean reverting as well?

Thanks in advance.