Google Mobile Services (GMS) are the apps by Google that often come pre-installed on Android devices. GMS is not a part of the Android Open Source Project (AOSP), which means an Android manufacturer needs to obtain a license from Google in order to legally pre-install GMS on an Android device. This license is provided by Google without any license fees.
GMS consists of two parts; a popular bundle package and an other bundle package. In order to gain a license for GMS, the popular bundle package needs to be pre-installed by Android device manufactures, which are usually called pre-loaded apps.
Source: Google Mobile Services (GMS)