Sometimes called a low-pass filter, this sits in front of a camera's sensor and limits the frequencies which can pass through. Frequencies past than the sensor's sampling rate cannot be recorded faithfully, which can lead to false patterns and colours being recorded in their place.
Setting a threshold in this way has a slight blurring effect on images, although the sharpening applied by cameras to images upon their processing can help redefine details. Anti-aliasing filters also have another purpose which is to prevent dust from reaching the sensor. Usually the filter is vibrated to dislodge any dust that may have got into the camera, and most also feature an anti-static coating to discourage dust from adhering in the first instance.