Synthetic fibres are man-made fibres and natural fibres are fibres that come from plants or animals.
Denim itself is not a fibre... Denim is a fabric which is mostly commonly made out of cotton fibres (which are natural). However today much more denim is being made from a cotton-synthetic fibre mix.
nylon is synthetic
Silk is a natural protein fiber that comes from silk worms
synthetic fibre = man maid products natural fibre = things that are made from nature