Mr. Williams is Mr. B's chaplain in Lincolnshire, which means he's working as a religious leader on Mr. B's land. When we first meet Mr. Williams, he expects that Mr. B will be conferring a "living" on him as soon as the person currently occupying that position has passed away. He then gets himself in hot water with Mr. B for trying to help Pamela escape, and so all those prospects go out the window. In fact, Mr. B has him briefly thrown in jail.
Mr. B is especially angry with Mr. Williams because he fears that Pamela is in love with him (of course, Mr. Williams did offer to marry Pamela to try to get her out of the Lincolnshire house, but she declined). After Pamela and Mr. B fall in love, Mr. B reconciles with Mr. Williams and even has him perform their marriage ceremony.
Brain Snack: a "living" was the church, house, and income that belonged to the estate of a wealthy landowner. To get a "living" meant that you'd be the rector of the church; it could be a pretty sweet gig, and they usually went to friends and relations of the landowners.
What all that means, practically speaking, is that Mr. Williams has a lot to lose by ticking Mr. B off. The fact that he's willing to help Pamela speaks highly for him: he apparently takes his religious duties seriously, which couldn't be said for all the Church of England clergy in the eighteenth-century. We don't know much about Mr. Williams, but we do know that we like him.