Tri-state is a bit like five people sitting around rocking a baby in its cradle, but each person wants to rock at a different rate, which we know, won't work well at all. If four of the people take their hands off the cradle (high-impedance state) and only one allowed to do the rocking (0,1,0,1,.....), the cradle will rock at a certain rate. You have your choice of which person rocks the cradle as long as that person does the rocking and the other four keep their hands off.
RAM and other memory is typical of 3-state logic (Tri-state is a National Semiconductor trademark) where only one RAM chip at a time is allowed to take control of the memory bus while the others keep their hands off (high impedance state).
Dean