Fallout 1st is monthly based subscription that costs 12.99 and month or 99.99 for a year and the benefits include

1650 Atoms per month
Private Adventure
Custom Worlds
Scrapbox
Survival Tent
Ranger Armor Outfit
Icons & Emotes Pack
bonus rewards from the season pass

Still confused on exactly what you're buying? dw I'll explain what every reward gives or does for you

This is a alternate mode that allows you to progress your character, items and progress without having other players in your world giving you the ability to pick and chose who can be in your lobby

The atoms you'll receive each month is 1,650, but what do you use these on? the real main options to spend these on is items for yourself or your camp inside of the atomic shop there are also other things like repair kits, lunchboxes and scrap kits which all have their own use

Scrap kits can take all the junk you have in your inventory and turn them into their base components and deposit them in your stash

repair kits are mobile kits that allow you to bring the durability of a item or armor back up to its amount of health without having to use junk to repair it

and lunchboxes provide a boost in xp and a neat effect when defeating a enemy

The secondary use for these atoms is available in the season pass where you can spend them to quickly get ranks without having to work for them

one of the other things you'll gain access to is the scrap box which serves essentially as another stash for junk that doesn't have a limit to how much it can have inside of it however junk is the only thing you get to put inside of the box

The survival tent is a mobile kind of camp you can place anywhere in the map that has a few things like a workbench, a stash and the previously mentioned scrap box but thankfully you also have the ability to fast travel to each one of your tents which you can place through the favorite wheel which can be accessed by using your scroll wheel or by pressing the favorite button on your dpad
How many tents can you have down at once? you can place up to 3 normally but after that the oldest tent will be removed for the new one to be placed

Fallout 1st will also give you one outfit which is the courier from fallout new vegas or "The Ranger" among other outfits that will sometimes be free

one of the other things you get is unique emotes or profile icons you can use in your respective areas but keep in mind you will need to go into the atomic shop and down to the icons and emotes section to click on and equip the profile icon or emote and you can see if it is equipped by a little checkmark that will be on it

Fallout 1st will also make some rewards that are locked behind fallout 1st in the season pass available as well as give you some bonus xp for completing challenges

There are certain times which are better to buy fallout 1st than at random due to fallout 1st being day based if you buy it on the 31st of a month when the next month doesn't have a 31st it has the chance of giving you fallout 1st until a month with 31 days and by theory if you buy fallout 1st on leap day you would get it for free until the next leap day

Thanks for reading