@Phaneendra S
Like debugging, this is a broad topic that is more an art than a science. To get you started, below are some links that provide insights on what you can look for.
This one is a presentation showcasing the "new" version of Explain and walks through a debugging example:
https://www.slideshare.net/HadoopSummit/how-to-understand-and-analyze-apache-hive-query-execution-pl...
This one explains how Hive Joins would appear in Explains:
http://www.openkb.info/2014/11/understanding-hive-joins-in-explain.html
As always, if you find this post helpful, don't forget to "accept" answer.