It's primarily upgrading the stock (as sold) ships thrusters, FSD, shields, weapons etc, it can also be reconfiguring the ship for a different role for example passenger transport rather than cargo or mining but that requires fairly significant changes to the loadout, have a look at these websites they're both tools that allow you to assemble different ship builds without laying out hard earned in game credits.
Coriolis
https://coriolis.edcd.io/
EDshipyard
http://www.edshipyard.com/
PS welcome to ED!