This is a textured polyester thread which is stretchy, resistant and puffy. It is used for sewing flexible fabrics such as jersey, Lycra and knitwear.
This Fil Au Chinois thread is called overlock or serger thread, because it is used on these machines. It is also extremely soft which is why it is used for underwear, swimwear, clothes that are worn next to the skin and washable baby nappies.
Attention: this thread can melt when ironed if you do not use a damp cloth.