Screeners (or Stock, ETF, Mutual Fund, Bond, etc Screeners) are a software that were once primarily used for stocks to help find a stock that corresponded to certain criteria. Nowaydays Screeners have been extended to all sorts of asset classes (even real estate if you think about it). There are large numbers of stock screeners and are available on most financial websites. More powerful screeners can allow to search with more criteria as well as more advanced and complex criteria.

As an example, if we wanted a very large company with high dividends. We could search in a stock screener for a high dividend yield and a large market cap.
Similarly if you wanted to start day trading you could look for stocks with a very high volatility and liquidity.
Most likely one of the most common searches are done to find favorable ratios (such as P/E, current and turnover ratios).

Given the huge number of stocks and ETF’s out there a stock screener is a must when trying out specific investment strategies.