There's 3 ranks of ship parts, A, B and C.
There's no requirement for A cert, you can always fly those ships, however some parts are more powerful to buy and those can also require a rank in the space ship design skill; the most powerful parts always require space ship design level 4 (max level)
The requirement for B cert is level 2 in the piloting skill, this will allow you to use ships that have a B cert reactor (the thing that powers everything on the ship) and also modify your own ships with a B cert reactor, so you can equip it with guns and shields and engines etc. that require a B cert reactor.
So the easy upgrade path is to put points in piloting and just upgrade to B cert, however the early B cert parts are actually weaker than the space ship design level 4 A cert parts, same with early C cert parts vs B cert space ship design level 4 etc.
The space ship design part isn't very important if you just want to do fast and easy upgrades, or just buy a ready to go strong ship, but if you for instance want to keep a small ship like the razorleaf, space ship design level 4 allows you to make it super strong, without making it bigger.