It somewhat depends on the context. But generally what is meant here is tightening both your abdominal muscles and also you back muscles to hold your torso stiff and rigid. You want to do this before picking up something heavy because you want a rigid torso to protect your vulnerable spine. Think about picking up a heavy box by squatting down, wrapping your arms around it, and standing up. If your back and abs aren’t tensed somewhat, your upper body will just fold like a pool noodle and you will do horrible things to your spine. When people say to engage your core, they mean you should really focus on getting that rigid core because it’s better to overdo it than to under-do it and hurt your back.
To address your question about fitness, no, unless you are literally missing those particular muscle groups, you can always use them to tighten your core. You can do crunches, planks, or any number of other exercises to strengthen your abdominals. You can strengthen your back by doing light deadlifts, good mornings, supermans, and by just consciously keeping an upright posture when sitting.
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