These are chemicals that are used in monopropellant thrusters on their own, without other chemicals, to produce thrust. Common examples include hydrazine, which decomposes exothermally in the presence of metal catalysts like iridium, and nitrogen gas which is used in cold gas thrusters
Chen, Xiaowei, Tao Zhang, Liangen Xia, Tao Li, Mingyuan Zheng, Zili Wu, Xiaodong Wang, Zhaobin Wei, Qin Xin, and Can Li. "Catalytic decomposition of hydrazine over supported molybdenum nitride catalysts in a monopropellant thruster." Catalysis letters 79, no. 1-4 (2002): 21-25.