In SQLmap, tampering can help you adjust the injection in specific ways required to bypass web application firewalls (WAFs) or custom sanitization mechanisms. SQLmap provides various options and techniques to tamper with the payloads being used for SQL injection.
Source: SQL Injection/SQLmap.md in PayloadsAllTheThings. From PayloadsAllTheThings by Swissky and contributors. MIT License, Copyright (c) 2019 Swissky.