Monochromatic (or mono) is a color scheme based on only one, single color tint. It uses only variations (shades) of a single hue, made by altering the saturation and brightness of the base color. Black and white colors are always added, as they in fact are the brightest, resp. the darkest shade of the color.
The result is smooth, elegant, comfortable for eyes, even for very aggressive base color. As there is only a single hue used, the is no clash of colors at all. However, it may be harder to find accents and highlights.
Examples
We can use the monochromatic scheme to create palettes with very similar, less contrast shades, as well as combination of high-contrast shades.