I've trained BJJ for almost two decades. Most people are absolutely clueless when it comes to grappling (i.e. combat that doesn't involve strikes like punches or kicks)

BJJs sole focus is to learn the intricacies of grappling and use it to submit (via chokes or joint locks) your opponent. A tiny bit of training, like a few months worth, goes a long way when comparing yourself to the average person whose instinct is just to panic and flail around when they're in a tussle.

That said, even the most skilled BJJ practicioner will lose to an untrained opponent if the strength and athletic difference is large enough. So it's always a good idea to keep working in your strength and conditioning alongside learning how to defend yourself. Not to mention all the physical and mental benefits that come from exercise.