I want a Fleet Escort Retrofit (I love the Saber class and I love small ships) but I don't know what Fleet Shipyard Tier 2 means. If it means I have to join a fleet, no thank you.
Also, what are fleet credits and ship modules?
Why can't you just buy it outright?
Yes, it means you need to join a fleet, at least to obtain it (fleet is in the name, after all). You can't buy it outright because that is how the dev team decided to let you upgrade your favorite ship to the highest tier.
The Fleet Shipyard is a part of a fleet's Starbase, you gain Fleet Credits by contributing to a fleet's projects, and ship modules are inventory items from the C-Store, created specifically for the purchase of Fleet ships.
It takes a LOT of time, resources, and grinding to build up a Starbase with a matching ship yard. Purchasing a ship also consumes fleet provisioning tokens, so unless you are willing to put some time into upgrading a starbase or replacing the provisioning tokens you will use in the purchase, getting that fleet ship will be challenging.