Digitizing a signal destroys all evidence of any frequencies above half the sampling rate, however samples of these higher frequencies map exactly into their lower frequency aliases. You cannot identify any of the higher frequencies in your sampled data, but you do detect each alias and this corrupts your results.

Here is a selection of anti-alias filter boards, which are installed between the sensor and any analog-to-digital converter. They effectively takes out unwanted higher frequencies and allow you to measure the true readings.