You are downloading GApps (Google Apps) for installing Play Store on your Android device with a custom ROM. If you do not install Gapps, you will not have Play Store for AOSP ROMs like CM12 or CM12.1! This Gapps package is compatible with Android 5.1 and 5.1.1.
What is MD5?
MD5 is a crypto-hash you can use to verify your downloaded files.
You can verify your downloaded file by using an MD5 File Sum Checker app! on your Android smartphone/tablet.
Virus and Malware Check
The following file has been checked for virus and malware.
Results: This file has no viruses and malware. It's safe to download.
File Size: 91.79 MB
Date Uploaded: April 26 2015 11:23:06.
Download Count: 4559193
Note: If you are downloading a zip file such as a custom ROM, make sure you do not unzip the file after downloading. CWM and TWRP recovery both use full zip files and if you unzip your files, the custom ROM will not install.