What does flush mean when I create new bucket?
The Flush option deletes every object in a bucket and is disabled (unchecked) by default.
It is used exclusively for one purpose: to enable your application to use the flush method and delete every object in the bucket.
Use the Flush option sparingly. It is highly recommended that you never run with this option enabled in production unless you absolutely must. It can be as dangerous as it sounds.
The Flush option flags the items for removal in a memcached bucket and immediately removes items in a Couchbase bucket.
Enable the Flush option either during the bucket creation phase or later, while changing the bucket settings. In both cases, after enabling this option you still must click on the red button Flush to proceed with flushing. Using this button is an additional security measure to prevent unintentional loss of data.
###Excerpt from https://developer.couchbase.com/documentation/server/4.1/clustersetup/create-bucket.html