This is short-hand recipe notation for how long to boil each hop, or how many minutes left in the boil until you add the hops.
So a 60-minute addition is boiled for 60 minutes, or with 60 minutes left in the boil, depending on how you think about it.
And a 0-minute addition is added when the flame goes out and you are no longer boiling, end of boil. For so-called "late hop" additions like this, it is common to delay chilling and keep the hops in there hot for at least 10-20 minutes and sometimes is called a "whirlpool" or "hop stand" addition. If kept hot for longer than about 10 minutes, the hops can add considerable bitterness as well as flavor and aroma. But if you chill down immediately and do not keep it hot, you will maximize aroma and minimize bitterness.
There are a LOT of different ways people add hops. The more you read and the more recipes you review, the more methods you will learn about.
Hope this helps as a primer to get you started on the right foot. Welcome to the forum!
Cheers!