Just as Microsoft keeps updating the Windows operating system (with Windows Vista, 7, 8 and 10, for example), Google keeps updating the Android operating system that's installed on mobile devices (such as phones and tablets). For some obscure reason, Google likes to use some rather odd names for the different versions of Android (or sometimes for groups of roughly similar versions).
The oldest version of Android that you might occasionally come across is 'Ice Cream Sandwich'. That was followed by 'Jelly Bean', 'KitKat' and 'Lollipop', with the latest version, 'Marshmallow', being released last October.
Some apps will only run on fairly recent versions of Android, so it usually makes sense to go for the most recent version, meaning that the phone with '5.1 Lollipop' might be the best option if you anticipate downloading lots of apps for it. However there are still plenty of users of KitKat and you shouldn't be too deterred from considering the phone using that version of Android if you're only likely to be using the apps which come pre-installed on it.
https://en.wikipedia.org/wiki/Android_%28operating_system%29#Platform_usage