Hadoop Common refers to the collection of common utilities and libraries that support other Hadoop modules. It is an essential part or module of the Apache Hadoop Framework, along with the Hadoop Distributed File System (HDFS), Hadoop YARN and Hadoop MapReduce. Like all other modules, Hadoop Common assumes that hardware failures are common and that these should be automatically handled in software by the Hadoop Framework.
Hadoop Common is also known as Hadoop Core.
Techopedia explains Hadoop Common
The Hadoop Common package is considered as the base/core of the framework as it provides essential services and basic processes such as abstraction of the underlying operating system and its file system. Hadoop Common also contains the necessary Java Archive (JAR) files and scripts required to start Hadoop. The Hadoop Common package also provides source code and documentation, as well as a contribution section that includes different projects from the Hadoop Community.