It doesnt quite work that way...its the relative strength index over however many days u choose...usually 14.
Under 30 is oversold and presents a buying opp and over 70 is overbought and presents a good selling opp....usually...its kind of medium to short term for trading. 50 is neutral since 0 to 100 scale.
If u already are holding shares then u wanna see it stay over 50 as much as possible. But if youre buying then u wanna see it under 30 or at least under 50.
However if youre at the bottom of a long term trend down or one just starting upward...even an 80 or so can be a good buying opp