A digital filter is a discrete time system that delivers an output, which is a modified version of its input.
Filters are the basic building blocks for most signal processing applications. They are typically used to extract or eliminate one or more constituent frequencies of an incoming signal.
Filters used for signal conditioning are usually designed from frequency response specifications and are called frequency-selective filters. Frequency-selective filters operate by attenuating some frequency components of the input signal while allowing other components to pass through unchanged. For example, a low-pass filter attenuates all frequencies in the input signal that are above a specified frequency.