Bucket field in Reports in Salesforce is used to group values to the name we specify.

It can group only the below data types fields

1. Picklist
2. Number
3. Text

Lets see an example.

Report without bucket field:

Adding bucket field:

Grouping Chennai, Coimbatore and Vellore cities to Tamilnadu and grouping Hydrebad and Nellore to Andhrapradesh:

Ouput: