What does to build a good will mean?

Actually, you don't need the definite article, since the expression "good will" (sometimes written as one Word, "goodwill") is non-countable, like such other English Words as "research" and "information." Building good will refers to creating a positive relationship, whether with customers or with your colleagues, so that over time, they come to trust you and they know you are dependable. It also refers to doing acts of kindness, which will create a good impression and make others think of you as a compassionate person.