Google has a concept called "labels" and has no concept of a folder. "Labels" are essentially tags that categorize messages. You may apply one or more labels to a message.
When migrating from Google, all labels are converted to folders. For every label that is applied to a message, a copy of the item is created in that folder. For example, if you label a message with A, B, and C, we will by default create the same item in three folders /A, /B and /C.
In addition to labels, Google has a folder called "All Mail" which contains a copy of every message in your mailbox. By default, we will only migrate archived email (i.e., email without any label) to the "All Mail" folder. However other options are available.