555-1 waitfor delay '0:0:15' --

What you pasted looks like a time-based SQL injection payload for SQL Server.

• WAITFOR DELAY '0:0:15' means: pause the SQL server for 15 seconds.
• The trailing -- is a SQL comment marker, so the rest of the line is ignored by the parser.
• The leading "555-1" on its own isn’t valid SQL; in an injection scenario it would be injected into a vulnerable query to trigger the delay.

Why attackers use this: by measuring the response time, they can infer whether the injected payload was executed, which helps confirm a vulnerability (time-based blind SQL injection).

Defensive takeaways (good for exam prep):

• Use parameterized queries/prepared statements instead of string-concatenated SQL.
• Avoid dynamic SQL; use ORM/query builders with proper binding.
• Apply least-privilege principle to the DB user used by the application.
• Validate and sanitize input; implement strict whitelisting where possible.
• Enable query timeouts and monitor for unusual delays or patterns that resemble WAITFOR DELAY usage.
• Consider logging and alerting on suspicious input patterns and on delays in responses.

If you want, I can give a concise exam-ready definition, or show safe lab-friendly defenses and how to phrase answers for an exam.