OBSELETE

This question is too wide open. What is your objective? If it were me I'd say nothing because I am still running Mavericks.

Do you want to upgrade? If so we cannot start to advise you without knowing the exact year and model Mac you are using. To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

Computer information as requested? Either that or DIY: How to get old versions of macOS - Apple Support