Black point compensation
The RGB color space can represent all luminosities from 100% white to 100% black. Output devices (such as printers and computer displays) often have limitations in how black the darkest black they can render is. Black point compensation attempts to resolve the difference between the darkest black in your photo and the darkest black that a particular device can render.
If no black point compensation is done, then detail will be lost in the darkest shadow areas of your photo because the dark areas beyond the darkest black the device can render are truncated.
On the other hand, if black point compensation is done, the luminosity range of your photo is compressed to fit into the luminosity range of the device. Hence, the darkest black of your photo becomes the darkest black that the device can render.
However, because the luminosity range is compressed, information from your photo is still lost: the loss is just evenly spread out over the entire luminosity range (and thus less noticeable) rather than being truncated at the black end.