This is the cotton stuff you use to create all the wonderful colours in the design. Floss comes in heaps of different colours and in some designs you need to combine colours.
Floss comes in 6 thread strands. Most charts specify either 2 or 3 thread stitches. If they don't it's up to you. Using 2 threads gives a delicate finish and using three threads gives a more textured finish.
Metallic and Rayon thread (shiny) are particularly difficult to work with because they fray easily and are very slippy so it's hard to keep the tension consistent. It helps to damp down this sort of floss to reduce the fraying and slipping.