You have your traditional relational databases/database server on one side and on the other, you have a Hadoop server.
How do you transfer the databases between the two, to-and-fro?
Here is where Sqoop comes into the picture. It's the bridging software between these two elements. What's special about Sqoop is that it uses MapReduce framework to import and export data. This provides parallel computing advantages and a higher amount of fault tolerance.
To learn more about concepts like Sqoop and master Hadoop, you can register for Hadoop Online Training.