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

That fragment looks like a time-based blind SQL injection payload aimed at Microsoft SQL Server.

What it means (high level):

• waitfor delay '0:0:15' is a T-SQL command that causes the server to pause for 15 seconds. An attacker uses this to infer information based on whether the delay occurs.
• The leading "555-1);" and the trailing "--" are attempting to terminate the attacker’s injected statement and then comment out the rest of the original query, so the injected WAITFOR DELAY runs instead.

Why this matters:

• If a web app builds SQL queries by concatenating user input, an attacker can insert such payloads to detect vulnerabilities by observing response times (time-based blind SQL injection).

Defense (safe, exam-focused summary):

• Use parameterized queries/prepared statements for all database access.
• Avoid dynamic SQL or sanitize/validate all input; never concatenate user input into SQL strings.
• Implement least-privilege database accounts (no rights beyond what the app needs).
• Use proper error handling to avoid leaking database error details.
• Employ input validation and escaping where appropriate, and consider ORM/query builders.
• Monitor and log slow queries, unusual delays, and attempted SQL payloads; consider WAF or security scanners.
• Practice in legal/controlled environments (e.g., DVWA, WebGoat, or vendor-provided labs) to learn defenses.

If you’re studying for an exam, I can draft a few short questions (and answers) about time-based blind SQL injection and defenses, or explain any specific part in more detail.