It can mean different things to different persons.
Having to do machining on a part you need fix it to the base. your design must ensure quick but secure setup.
Having to do a small opp on a lot of parts the time wasted by fixing would be excessive. In that case you may make a jig where your part can fit or snap into. and keep it down with a spring loaded clamp.
A jig may also be a guide for where holes should be drilled or . . ..
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Never do today what you can put of until tomorrow - Student motto