The Nyquist rate or frequency is the minimum rate at which a finite bandwidth signal needs to be sampled to retain all of the information. For a bandwidth of span B, the Nyquist frequency is just 2 B.
If a time series is sampled at regular time intervals dt, then the Nyquist rate is just 1/(2 dt ).
When searching for periodicities in a time series, frequencies greater than the Nyquist rate get attenuated and appear at lower frequencies than they are really present in. This is known as "aliasing".