Circumstantial evidence means that the person is being accused based on evidence that cannot be backed up. In other words, circumstantial evidence means that you can infer that the person did something wrong, but you can't prove it outright.
In Tom's case, he went into Mayella's house, so he was there around the time that she was attacked. Also, Bob Ewell saw Tom and Mayella kiss, which was definitely not allowed back then. That doesn't mean that Tom was the one who attacked Mayella, but that is how the prosecution makes their accusation. Tom's "guilt" is based on his presence in the Ewell home, the kiss, and his race. The jury feels that he should not have been in a white woman's home when she was home alone, and they also feel that since he is African-American he is more likely to hurt someone or try to rape Mayella than anyone else would be. They do not consider that Mayella kissed Tom, but assume it was the other way around. This is why they convict him, even though they do not have actual proof that Tom hurt Mayella.
Circumstantial evidence is "indirect" evidence, which means that there is no direct evidence, such as a witness to a crime. Circumstantial evidence would be a witness to the aftermath of a crime, such as seeing a person holding a gun and a dead person on the ground in front of him. That person (holding the gun) could have come upon the scene and (stupidly) picked up the gun. It doesn't mean he shot the victim. In chapter 23 Atticus says he did have misgivings about them giving the death penalty based on circumstantial evidence.
"I mean, before a man is sentenced to death for murder, say, there should be one or two eye-witnesses. Someone should be able to say, 'Yes, I was there and saw him pull the trigger.'"