CMYK is known as a subtractive process, meaning that for each additional colour used, more light is removed to achieve that colour. When using 100% values for each colour in CMYK, the result is black. By its nature the colour profile for CMYK is limited in comparison to the RGB colour profile (what you see on computer screens). RGB results in a more vibrant and diverse colour profile than CMYK.
CMYK can be used in screen printing to create a full colour design, although there are more considerations and complexities when screen printing CMYK on fabric than printing CMYK on paper, which will be outlined below.