A camera sensor is sensitive to both the visible portion of the electromagnetic spectrum and the Near Infra-Red (NIR) portion of the spectrum.
Electromagnetic Spectrum
This feature allows the sensor to form images even under low illumination conditions that a human eye would find difficult to do. However, this sensitivity to the NIR segment of the spectrum results in distorted colours during daylight illumination; therefore, the camera sensor requires an NIR filter (or IR-Cut filter) to be placed between it and the lens, during the day. At night or during low-light conditions, the NIR filter becomes an impediment, in the formation of good quality images, and therefore needs to be removed.
The ICR (IR-Cut filter Removable) is an apparatus that is placed between the camera sensor and the lens, and allows the IR-Cut filter to be placed or removed from the front of the sensor, depending on the illumination level. The ICR is a mechanical shutter, typically powered by a motor, with NIR filter and plain glass panes.