As its name indicates, it is a fiber obtained from synthetic material. To spare you the multitude of quadrisyllabic molecules that can come into play depending on the fiber you are hoping for, these chemical compounds come almost exclusively from hydrocarbons. In other words, a garment made of essentially synthetic material is plastic in the same way as a plastic bottle. These fibers may be strong, but they do not allow the fabric to breathe, which means it has to be washed more often, and therefore degrades faster.
At Adalma, we avoid using these materials as much as possible, or we use them exclusively for their very specific property in minimal percentage (elastane for elastics).