Frigates in No Man’s Sky are a part of your freighter fleet. You recruit them by going to different star systems and simply paying to have them join your fleet. They come in different classes such as combat, industrial, exploration, trading & support. It’s important to have at least 5 support frigates in your fleet because they reduce the cost of fuel it takes to complete a mission. They are not necessary but certainly helpful.
Frigates also level up by doing missions. You don’t have to do anything there, simply keep sending them, and eventually they will all become S class.
You can have up to 5 missions per day and also don’t forget to construct 5 control rooms for each mission to utilize. When recruiting ships for your fleet, add a mix of all classes and at least 5 support vessels.
They can also have different technologies installed while on a mission (temporary upgrades) and you research those in the freighter technology research panel. Those are also not necessary but potentially useful.
Since getting your first freighter is free, make sure to initially research freighter fuel because without that, you can’t send any frigates on a mission. I also recommend maxing out the ships per mission. While you can send one ship on each mission, it will be much better (safer & more profitable) if you send the maximum number of ships that you can.
Frigates can also have different traits and bonuses (and negative ones), but those aren’t that significant so don’t worry too much about them. You can certainly inspect all of that before recruiting them, so if you see something that you do not like, simply don’t recruit that one.
If one of your vessels gets damaged, make sure you have some basic materials in your inventory, go land on the ship and talk to the NPC to get the damaged components marked for repair. You have to physically go there and repair them yourself.