Complex sentence: A mordant is a "substance used to set dyes on fabrics" by forming an insoluble compound with the dye or an "acidic chemical used to etch away areas of a plate" which are not protected by another material called a ground.
Split into simpler sentences: A mordant is a substance used to set dyes on fabrics by forming an insoluble compound with the dye. Mordant can also refer to an acidic chemical used to etch away areas of a plate which are not protected by another material called a ground.