The term "mat up" is a verb phrase that describes the process of a material, usually fabric, becoming matted or felted together, often due to washing or wear and tear. When something "mats up," it means that the fibers in the material get tangled and stick together, making it feel dense or fuzzy, similar to felt.
Overall, "mat up" is a useful term especially in discussions about textiles, laundry, and crafting.