harry is a command line utility that will convert a HTTP Archive (har) into a JMeter test plan (jmx)
JMeter is a tool used to test the reponses for a given HTTP request. However, a typical webpage is comprised of dozens of such requests. To accurately test the performance of a webpage with JMeter, you would need to perform a great deal of manual entry.
HTTP Archives are great for recording all of the requests made on a page - exactly the data one would need to replicate all the requests for a JMeter test.
This reads in a HTTP Archive, spanning one or multiple pages, and generates a JMeter test plan to replicate the requests. This can then be directly used by JMeter to begin testing.