Miss Maudie says, “We’re paying the highest tribute we can pay a man.” What does this mean?

I think you need to examine the wider context of the quote:

"Have you ever thought of it this way, Alexandra? Whether Maycomb knows it or not, we're paying the highest tribute we can pay a man. We trust him to do right. It's that simple."

"Who?" Aunt Alexandra never knew she was echoing her twelve-year-old nephew.

"The handful of people in this town who say that fair play is not marked White Only; the handful of people who Pay a fair trial is for everybody, not just us; the handful of people with enough humility to think, when they look at a Negro, there but for the Lord's kindness am I.” Miss Maudie's old crispness was returning: "The handful of people in this town with background, that's who they are."

Miss. Maudie says that Atticus means something to the progressive white people in the town. Atticus represents change for the positive even though he lost the Tom Robinson trial. All the people, ignorant and progressive, still know the truth because of Atticus.