She is sitting at a table.
She is sitting on a table.
If you are sitting on a chair eating from the table, you are sitting at the table.
If you are sitting on the top of the table, you are sitting on the table.
She is sitting at a table.
She is sitting on a table.
If you are sitting on a chair eating from the table, you are sitting at the table.
If you are sitting on the top of the table, you are sitting on the table.
Again great example easy way to learn english