I am a musician in a band and we want to record a demo without buying studio time. I've been looking at multitrack recorders on Ebay, but I'm not sure exactly how the work. Could someone explain to me in simple terms?

I'm a musician in a band and we want to record a demo without buying studio time. I've been looking at multitrack recorders on Ebay, but I am not sure exactly how they work. Could someone please explain to me in simple terms?

Thank you