Using a mountain or pile of stuff as a metaphor (and ignoring its potential lopsidedness for now): At the base, you've got some knowledge that might be kind of

Using a mountain or pile of stuff as a metaphor (and ignoring its potential lopsidedness for now): At the base, you've got some knowledge that might be kind of dusty, but that you can reacquaint yourself with quickly. Then you've got a mass of experience, layered on top of that knowledge, that further colors how you'll approach problems, the solutions you'll choose, etc. You can reach further from there by doing a little research, and put another layer of experience on the pile you already have. If the job doesn't require any reach, it's "burger flipping". If the job requires more reach than you can get, even from the top of your experience-pile, and building the pile up to extend your reach enough to achieve the goal will take more time than you have, then you're unqualified for the job. So, you want something in the happy middle: not trivial for you, but not out of your grasp. Your reach should be small, compared to your experience, unless you're just starting out.