A color filter used in black and white photography to change the shade of grey that colors are reproduced as. They’re called ‘contrast’ filters because they can change the contrast (in shades of grey) between different colors.
Contrast filters are usually round filters that attach directly to the filter thread of the lens. They are less popular than they used to be because camera sensors work differently to black and white film. They record in color regardless, so it’s as easy to change the color to greyscale rendering when they are processed as it is to attach a filter to the lens. In fact you have more control if you do it later.
The exception is those rare digital cameras that have monochrome sensors with no color filter array. Examples include the Leica M11 Monochrom and Pentax K-3 III Monochrome.