This feature is accessible from the More Effects dialog.
This reduces the number of colors used by the image or selection, with or without dithering.
Dithering generally provides better quality results, however, you may wish to disable it for artistic effects; e.g. using Monochrome instead of Monochrome (Dithered) gives a silhouette effect.
Another important distinction is that while Monochrome (Dithered) will always reduce the entire image or selection to black and white, Monochrome will do this only if the image or selection contains more than 2 colors.
For a quick, dithered monochrome image or selection, use the Reduce to Monochrome (Dithered) item of the or menu. The item is only displayed in the menubar if you use one of the selection tools. Additionally you can reach this action from the context menu opened with a mouse button click in the image area.
Changing the number of colors here has no effect on the color depth of the file format. If you want to change the color depth, you should select it in the file saving dialogs. Note that, confusingly, changing the color depth also changes the number of colors.