Primary jobs are the things that are the biggest responsibility, the ones that take priority. Secondary jobs are the ones that they help out with/are learning.
In Ash's case, he concerns himself with Engineering as a job, mostly. But he also runs the Distillery. Engineering is more important, but he won't always have a project that demands his attention. Nor does he have to spend a huge amount of time everyday in the Distillery making sure everything is okay, it's more of a "set up and check in" kind of thing, with one or two days per series of batches of multiple hour work.
Primary and secondary rate priorities. You'll be spending a lot more time in primary than secondary, except under specific circumstances. It's also how you learn stuff, unless you can find/bribe someone to teach you during off hours. Unless given direct instruction to handle something, feel free to have your character jump into whatever duty or study works for you creatively. Make sure they're doing their jobs, and have fun with it.