The backup retention is the number of backups you want to keep available.
How does backup retention work?
Every time a backup runs, we check the number of backups you already have stored and if this number is higher than the retention value we delete the oldest backup files still stored.
When should I use backup retention?
A good practice is to always define a retention value, in order to avoid using too much storage.
The value will mainly depends on the context of the project you're backing up.
When you created your backup, ask yourself the question "Till how long back in time do I need to be able to go if something goes wrong with my site/database"?
Good to know
Another great way to approach it is to define multiple backups for a given project, one with a 7 days retention scheduled on a daily base and a second one with a retention set to 12 scheduled on a monthly base.
This will allow you to travel 12 months in the past with a low granularity (1 per month) and have a daily backup granularity for the last week.