If you're using a custom ROM on your device or planning on flashing one in the near future, Gapps is something you're probably already familiar with, and you'll need to flash it to your device post ROM flash if you want access to the standard Google services apps like Gmail, Chrome, Calendar, the Play Store and others. Without it you won't be able to load apps onto your device, and as versions of Android change a new version of Gapps should be on your radar as you'll want to have the most up to date version and you'll want it to match up with the software you have flashed to your phone or tablet.
The latest version of Gapps is for Android Lollipop 5.1, so if you have already flashed a ROM to your device that's built off the 5.1 software, picking up this version of Gapps if you don't already have it on your device should be a priority unless you plan to be without the Google applications and services. This version of PA Gapps comes in multiple different variants including a full build of the Gapps, a stock build, a mini build, a micro build, a nano build, and pico as well as pico (uni) builds. All generally serve the same purpose, which is to get the standard Google services like the Play Store onto your device.
Each build is targeted specifically at a set group however so each build comes with different traits, like the pico (uni) build which was designed to be universally compatible with multiple screen DPI variants. You can find all the download links for the PA Gapps 5.1 at the XDA page here, but remember if you haven't installed any custom ROMs on your phone this won't apply to you. Furthermore, if this will be your first time flashing any Gapps package to your device, make sure to follow all instructions located in the XDA thread for the files. If you're interested in knowing all the details and differences for each build of PA Gapps, the chart below lists off all of that information for you.