The same thing: a filter that only lets through low frequencies. A blur effect is a spatial low-pass filter. A motion blur effect is a temporal low-pass filter.
What GP is talking about is the antialiasing low-pass filter used when downsampling video to smaller resolutions, or when initially sampling it at the sensor. This is the same exact concept as the antialiasing low-pass filter used when downsampling audio to lower sample rates, or when initially sampling it at the ADC.
