The base

Can you help me to understand to what "the base" refer here?

While the temperature was maintained at around 50 degrees Celsius, the mixture was stirred for about 30 minutes. In this step we're doing what's known as a transesterification reaction, and we're liberating the fatty acids from the glycerol backbone. The base acts as a catalyst and for every triglyceride we produce 3 methyl esters and one glycerol molecule.