A stock screener is an analytical tool that helps an investor filter out stocks that match the selected criteria (or investment strategy) from the larger pool of stocks. You can select the parameters that you want to use - fundamental or technical -  to identify stocks that match the conditions that you have set. 

Stock screeners can be used to choose stocks based on price, market capitalization, valuation ratio, profitability ratio, return ratios, FnO indicators like open interest, call or put volumes, moving averages, etc.