555wgZWSKJS'; waitfor delay '0:0:15' --

That looks like a time-based SQL injection payload intended for Microsoft SQL Server.

What it is doing

• 555wgZWSKJS' ends a string literal in SQL (the trailing apostrophe closes a quote).
• ; ends the current SQL statement.
• waitfor delay '0:0:15' tells SQL Server to pause for 15 seconds.
• -- starts a comment, so the rest of the original query is ignored.

How it works conceptually

• If an application builds a SQL query by concatenating user input (e.g., "... WHERE name = '" + userInput + "'"), an attacker can inject a payload like this to terminate the original string, insert a new statement (the waitfor delay), and comment out the rest.
• The server’s response time increases by approximately 15 seconds if the injection is successful. From this, an attacker can perform blind inference (e.g., boolean checks, data extraction) based on whether there was a delay.

Defensive measures (what you should study for exams and practice)

• Use parameterized queries / prepared statements everywhere. Do not concatenate user input into SQL strings.
• Validate and sanitize inputs, and prefer allowlists (whitelist only expected values, especially for IDs, enums, etc.).
• Avoid dynamic SQL construction. If dynamic SQL is unavoidable, use proper parameter binding for user-supplied values.
• Apply the principle of least privilege to the database account used by the application (limit permissions, disable dangerous features like xp_cmdshell on SQL Server if not needed).
• Use ORM frameworks that generate parameterized queries.
• Implement input/output encoding where appropriate, and separate code from data.
• Timeouts and monitoring: set sensible DB and application timeouts; monitor for unusual delays or patterns indicative of blind SQL injection.
• Web Application Firewall (WAF) and IDS rules to detect common SQLi payloads and slow/timeout anomalies.
• Secure development/testing practices: test in a controlled lab with permission (e.g., DVWA, OWASP Juice Shop) and learn how to reproduce safely.

If you’re studying for an exam, you might see questions like:

• Explain what a time-based blind SQL injection is and why it is dangerous.
• Compare blind vs. classic (error-based) SQL injection.
• List mitigations to prevent SQL injection in web apps.
• Describe how to configure a safe database access pattern in code.

If you want, I can give you a short, exam-style practice question with answers focused on identifying vulnerabilities and proposing mitigations.