FIFO means First In First Out.
Technicaly, FIFO does nothing, except adding numbers to cards in first column. The main goal is to make transparency between tasks priority in different swimlanes.
For example, you can use different swimlanes for your internal customers (ex: Lawyers, Marketing and Sales), then you can set different limits for each lane (Marketing can add 3 tasks, but Sales only 2).
This is the good approach to balance interests, but what task you need to do first?
Here is FIFO helps you. Everytime take task number 1 (when it goes to next column, all numbers will reduce by one).
Sometimes, you may need to work on urgent tasks in the first queue. Just set up separate swimlane for them.