Define your shipping vendors and their respective shipping methods. These settings are used by (a) integrations e.g. Shipstation, EasyPost etc. (b) manually record how the shipment was carried.
Use if you need to setup records for the commensurate list of carriers and their available shipping methods. These values are passed via integrations e.g. EasyPost to the carrier's systems when you request rates.
You set up your Shipping Carrier(s) in Configuration>Shipping Carriers.
Add the name of the carrier, plus the shipping methods separated by a semi colon (;), e.g. Express;Next Day;Overnight.