Everytime I see live acoustic shows when they are plugged in, I see the guitarist has some black thing inside the guitar under the strings, what is it and whats it's purpose?
I'm gonna take a blind guess and say you're new to guitar?

That part isn't important though;

The black thing underneath the strings is a pickup, like what you'd see on an electric guitar. There's a whole world of them out there too. Start looking up Acoustic pickups on google if you're interested in one
It could also be a feedback buster. They block the sound from coming into the cavity, resonating the guitar body, and causing it to feed back.
A simple piece of cardboard or plastic & sticky tape will do the same job (in a pinch).

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