Welcome to the forum, @Anu1!
I haven't used gneiss, but here are some resources that might get you moving in the right direction:
A couple of notes
- this visualization is deprecated, and will be removed in a future version of QIIME 2.
- while your question is clear and reasonable, it is also very general. As you read up, try to frame more specific statements of what you do and do not understand. That'll help ensure you get the answers you need.
Good luck, and happy 
-ing!
Chris