In your Backlog you can break down large tasks, stories or ideas into new cards. This can be very useful if you realize a card subject is far too large to be handled well.
In the above image, you can see a backlog that contains tasks that need to be done in preparation of a new team member joining. The round of introduction card can be easily handled a bit ad-hoc and needs no further break down. If a break down is needed though, you can click the plus symbol to the right to accomplish this.
The Office essentials card contains five more cards. Clicking on the number and arrow reveals the below list.
As you can see, the Office essentials card has been broken down into five new cards. You can see in the header that you are currently in a card break down. To return to the main backlog, you can click on the parent list. You may also notice that the cards in this sub-backlog also have plus symbols to the right. You can have an infinite amount of levels if this is needed for your process.