Bucket testing is a technique to identify the impact of New product design or in other words we can say New UI on a website benchmark.
In Bucket testing two different versions of single or set of webpages are run. The motive is to find out difference in click , traffic and transaction between the two versions.
Bucket testing gives user a way by which user sends small amount of traffic (generally less then 5%) to a different user interface. By this we can make sure that if our new Interface has some drawback then it will not impact to the large traffic of our website.

Example :
Lets take a scenario where you have a site (www.view.com) which is getting millions of page view each day. In this case a major change in the UI or functional area can hamper the view of our site, if it doesn't go well with the users. And it can also cause the users to go away and they may never come back.
So we will first introduce the new design to the small percentage of the user, let say 5% , and will run the New UI for certain period of time. And in the mean time system will collect the data for the five percent bucket. And we will analyse the data and will confirm if to confirm the change to the whole world.