A bucket is a special item in Sitecore which is a container for other items.
For Example, Sometimes You have a number of items which has a common template and storing in one place and sometimes it is hard to find a specific item based on the name so the bucket is the best solution to use if your item name starts from a then you just create a bucket with the name of A and store all the item which name starts from A. Please Find Attached a screenshot below for your reference.
What is an Item Bucket?
Item Bucket is useful to store and manage many content items without loading them into the content tree. Item Buckets can be created for a new content item or converted to an Item Bucket in an existing item structure.
Default item bucket
Also You can store the items with the alphabetical Order into Bucket.
As I’ve created an item with My New Blog name. it has created bucket M as First Level and Y as Second Layer and under that bucket out blog item created.