Lanes are used to segment a board horizontally, giving you an additional dimension for categorizing cards on the board. They are commonly used in the Kanban methodology, in particular for establishing an "expedite lane" for urgent work. Lanes can be given many of the same attributes as cards, such as tags or image attachments.
You enable lanes from the App Store in the hamburger menu of your board.
An example use for lanes is if you have two teams working on the same board but you want to be able to control WIP limits for both of them. Below you can see such an example.
With both teams working in the same board, the teams has an easier time to control WIP limits since all tasks are in the same column and board.