This is already answered by sciftw at:
It is an excellent explanation.
This is a great scene in the book. Jem and Scout are present as well for this scene, which makes it a reliable telling of the event. It is not something that Scout heard happened, but it is something that Scout witnessed.
The full text is as follows:
"To our amazement, Reverend Sykes emptied the can onto the table and raked the coins into his hand. He straightened up and said, “This is not enough, we must have ten dollars.”
The congregation stirred. “You all know what it’s for—Helen can’t leave those children to work while Tom’s in jail. If everybody gives one more dime, we’ll have it—” Reverend Sykes waved his hand and called to someone in the back of the church. “Alec, shut the doors. Nobody leaves here till we have ten dollars.”
The church congregation is a black congregation. That means the church is full of people that know Tom Robinson and his wife. They know that he is in jail. They know that Tom cannot financially support his wife and children from jail. They know that Helen Robinson is struggling to put food on the table.
At first Scout doesn't understand the need for a special collection. She thinks that Helen could work the fields like all of the other black women. What Scout doesn't understand is that nobody wants to hire Helen, because she is the wife of an accused rapist. Reverend Sykes knows all of this and wants to help out the Robinson family, so he takes up the special collection. He also knows how much money is needed, and he is not willing to let his congregation go until enough money is donated to the Robinson family. That is why Reverend Sykes has the doors closed and demands that nobody leaves until the 10 dollar amount is met.
What the entire scene shows how the black community is willing to rally around Tom and his wife Helen. The black community was not going to be a group of people that supported only with their thoughts and prayers. They were going to be a community that supported with action. In this case, their action is to financially support Tom's family while he is unable to do it.