"Thick fabric" refers to a type of cloth that is heavy, dense, and has a lot of material woven or knitted together. It is not thin or light, but rather is sturdy and provides insulation or protection. Full definition
They can be made of thick fabric so you can have less light penetration in your room or a thin material so the light can penetrate a little more. (thefashionablehousewife.com)
This jacket runs a little small, so order one size up to allow for thicker fabrics from your fall clothing. (bayareafashionista.com)
The dress has long sleeves and a moderately thick fabric so whether it is spring, autumn, or summer, you will feel comfortable and beautiful in this piece. (dresshead.com)