The quote in question comes from chapter twenty-one from Harper Lee's novel To Kill a Mockingbird.
The quote is spoken by Scout as she watches the jury come into the courtroom to hand down Tom Robinson's verdict.
What happened after that had a dreamlike quality: in a dream I saw the jury return, moving like underwater swimmers, and Judge Taylor's voice came from far away and was tiny. I saw something only a lawyer's child could be expected to see, could be expected to watch for, and it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing that the gun was empty.
This is a simile. A simile is a comparison between two things which uses the words "like" or "as" to define the comparison being made.
Therefore, given the quote uses the phrase "it was like watching Atticus," the quote is an example of a simile. The comparison being made is between Atticus with an empty gun in the street and the handing down of Tom Robinson's verdict. Scout is explaining that she feels (now in court) the same way she would have felt if Atticus would have gone into the street to shoot the rabid dog with an empty gun.
For Scout, the whole world seemed to stop and change when Atticus shot the rabid dog. Here, Scout is admitting that something is about to change again.