Think of bias as an adjustable offset of a particular variable. A linear +/- adjustment. With that in mind, review the specific algorithm for the control system. The description may differ somewhat.
In simplicity, bias control is "knowingly offsetting a measured value". Suppose you have a temperature control system which you want to set at 100C. Now you have some doubt about 100C reading so you send your instrument guy out in the field and he checks the temp. and says it's 120C. After conforming that your controlling temp. is 20 degrees higher than what it shows on your DCS/controller you set your controller at 80C (-20C bias) to make it control the temp. at 100C.
