Oxidizer is a chemical that will allow fuel to burn. It may combust with fuel spontaneously or require an ignition source. Liquid oxygen is the most common oxidizer used in launch vehicles. (Oommen and Jain 1999)
Oommen, C., and S. R. Jain. "Ammonium nitrate: a promising rocket propellant oxidizer." Journal of hazardous materials 67, no. 3 (1999): 253-281.