Rayon fibres are perhaps best know for their use in Brazilian Embroidery, but they can be used in other techniques as well.  For most other types of needlework you would probably just want to use them for a specific area, rather than stitching the whole piece in rayon.
Rayon is a man-made, synthetic fibre, which gives the appearance of silk, while being more economical.  Slight color variations may occur due to hand dye process which is being done in South Africa.  Wash in cold water