YARN stands for Yet Another Resource Negotiator.
YARN was developed by Yahoo in 2010 as a substitute of Map Reduce 1. It was able to overcome disadvantages which were present in Map Reduce 1.
In Map Reduce 1 Job Tracker was used to both the Job scheduling and Task Monitoring. This lead to the bottleneck in scalability of the cluster. YARN separate this two roles into 2 separate daemons which are Resource Manager and Application Master.