Please explain stamina?

Hi all! I'm new to the forum. You guys have a great community here! I'm hoping someone can help me understand how stamina works. I don't understand what it actually does (how does it help?) and when or if I should 'use a stamina drink to restore one stamina point.' I've searched everywhere, but if I've missed a thread that explains it, feel free to redirect me. Thanks so much in advance!