RSI can be used in many ways.
A Price Action trader may use it to evaluate price movements at support/resistance levels. (e.g. a return back through the 70 or 30 level)
A Technical Analyst may use it to indicate where a price move is reaching exhaustion level and exit positions (as RSI exceeds 70/80 on the upside or 30/20 on the downside)
or another may use it as a secondary signal confirming, or otherwise, a primary signal such as an MA cross.
another may use it to highlight divergence between high/lows in price and high/lows in the RSI line.
You might like to read through this link to help grasp these and others:
RSI is a very common indicator and there is a mass of info on Internet on ways it can be applied to and associated with other indicators.