As I discussed a year ago, fast repetitive signals or those modulated on a high frequency carrier are fairly easy to filter. An example is your TV set, which receives not much more than a few microvolts at the antenna, buried in many millivolts of noise. This is easy to filter since the signal is modulated on the station's carrier frequency. Narrow bandwidth RF filters notch out virtually everything that is not at almost exactly the carrier's center frequency. Radios are rather a special case.
Most signals presented to digital systems are at a much lower frequency.
Indeed, most are non-periodic, nearly DC voltages. It's important that only the latest N readings are included in the average, so very old data can't skew the results. N is chosen large enough to generate quiet data, but must be small compared to the rate data changes at, to avoid smearing the results. For a running average, as each sample is collected the oldest is dropped and the new one added. A circular buffer is often used to store the readings.