Search is not working for partial words. I have a phrase ‘Backfield Investigation Reporting’. If I am searching the fields with a search query ‘Backfield Investigation Reporting’ it returns me the correct result even if I am typing the Backfield. But when I am tying the 'Back' it does not return the correct result.
Just like Trevor said, wildcard character will help you. For more information about the Keyword search query, please check this: Keyword Query Language (KQL) syntax reference
It will help you better understand and manage your search query and construct KQL queries for Search in SharePoint. This syntax reference describes KQL query elements and how to use property restrictions and operators in KQL queries.