Heck Tate says he was covering Atticus the entire time the mob was there, meaning that he would have shot anyone that tried to hurt him.
There is a group of men who are trying to kill Tom Robinson, Atticus’s Negro client, to prevent a trial. Mr. Heck Tate, the sheriff, tells Atticus they are moving Tom Robinson to the county jail and he does not want trouble.
"Don't be foolish, Heck," Atticus said. "This is Maycomb." (ch 15, p. 102)
Yet Atticus decides to sit outside the jail and protect Tom, in case anyone does try to hurt him.
Mr. Underwood is the sole proprietor of the Maycomb Tribune, the newspaper. He gets most of his news from looking out the window, and “something must have been up to haul Mr. Underwood out” (p. 104). This is one of the reasons that Atticus was safe.
Mr. Underwood and a double-barreled shotgun were leaning out his window above The Maycomb Tribune office. (ch 15, p. 110)
When Mr. Underwood says he has had Atticus covered the entire time, this is important because it shows that Atticus is still cared about by some in the town. It also demonstrates that Atticus was brave enough to stand outside, even though the sheriff was not there, to protect Tom Robinson.
His comment is important in the novel because, although his character carries many predjudices against blacks, he supports Atticus' defense of Tom at the courthouse. Even Atticus himself says that Mr. Underwood "despises Negroes" which makes his actions all the more surprising and noteworty in the novel.