This quote from The Scarlet Letter, which was written by Nathaniel Hawthorne, refers to Roger Chillingworth, who is the antagonist (villain or "bad guy") of the novel. Essentially, the quote is intended to clarify Chillingworth's role in the life of the Reverend Arthur Dimmesdale.
By saying that Chillingworth is "not a spectator only, but a chief actor, in the poor minister's interior world," Hawthorne is giving the reader a considerable amount of important information regarding Chillingworth's presence. Chillingworth is not just a spectator, which means that he will not simply be watching or observing Dimmesdale without interfering in his life. Because Chillingworth is referred to as a "chief actor" in Dimmesdale's life, the reader knows that Chillingworth will play an important role in what happens to Dimmesdale throughout the novel. Finally, the minister's "interior world" refers to his intimate world.
Basically, this passage by Hawthorne means that Roger Chillingworth was not content to watch Arthur Dimmesdale; Chillingworth became very important to Dimmesdale's private personal life.