The document discusses how SQL injection vulnerabilities can allow attackers to hack into websites. It begins by explaining that SQL is used to access and manipulate database data and that many websites store user login credentials in a database. It then shows how submitting malicious SQL code as input can allow an attacker to bypass authentication by manipulating the SQL query used to validate login credentials. Specifically, it demonstrates how adding OR clauses to the username or password fields allows logging in without valid credentials. The document also explains how error messages from failed SQL queries can be used to fingerprint the database structure and design further attacks.