Tell me more about SQL injection?

SQL injection is a code injection technique, used to attack data-driven applications, in which malicious SQL statements are inserted into an entry field for execution

SQL injection is one of the Web hacking technique to destroy the data bases by the placement of malicious code

Apart from the above you can get the detailed information from the following

https://www.w3schools.com/sql/sql_injection.asp

SQL Injection (SQLi) refers to an injection attack wherein an attacker can execute malicious SQLstatements (also commonly referred to as a malicious payload) that control a web application's database server (also commonly referred to as a Relational Database Management System – RDBMS).