What is the SQL INJECTION?

Answer 1

In my own words SQLI means parsing codes to tamper with a database against its original programming

Like

SELECT * WHERE id = <input name> AND password = <password>

One can parse codes to read

SELECT * WHERE id = admin OR 1 = 1 AND password = admin OR 1 = 1

Answer 2

AFAIK A method to break in a website's db by inputing char/string to make the db error and inject it with sql queries. forexample ( ' ).