I actually Don't understand what's the difference between RAM, MEMORY and BANDWIDTH here... I Couldn't find any information about this in Google Cloud Docs.
Is Bandwidth and Memory same thing??
No, Memory and Bandwidth are not the same things on GAE. Bandwidth is how much data will be processed by your application. You can get more information on bandwidth on this Quotas page.
Instance Memory is the cache data stored locally by GAE. You can get more information on limits of Memory on GAE on this other post: What is the maximum memory a Google App Engine app may consume?.
RAM would be how much memory RAM you want to have for your application run. This is configured in the app.yaml file.
Let me know if the information helped you!