What is the Nyquist sampling theorem?

The Nyquist sampling theorem states that to accurately reconstruct an analog signal from its samples, the sampling frequency must be at least twice the highest frequency component in the signal (fs >= 2*fmax). This minimum rate is called the Nyquist rate. If sampling is below this rate, aliasing occurs where high-frequency components appear as lower frequencies, causing distortion that cannot be removed. Anti-aliasing filters are used before sampling to remove frequencies above fs/2.