-1' OR 5*5=25 --

This looks like a classic SQL injection payload.

What it does (high level):

• -1' ends a string literal in the SQL query.
• OR 55=25 injects a tautology (55=25 is true).
• -- starts a comment, so the rest of the original query is ignored.
• The resulting SQL can become something like: ... WHERE username = '-1' OR 25=25 -- ... which is always true, potentially bypassing authentication if the input is concatenated into SQL without safeguards.

Defenses you should know for exams:

• Use parameterized queries (prepared statements) instead of string concatenation.
  • Example (conceptual): SELECT * FROM users WHERE username = ? AND password = ?
  • Bind the username and password as separate parameters.
• If you’re using an ORM, rely on its parameterization rather than building SQL by string concatenation.
• Validate inputs with allowlists and proper types; avoid trusting user input.
• Use least-privilege database accounts (the app should only have access to what it needs).
• Consider stored procedures with parameter binding, and disable unnecessary features that could be abused.
• Don’t reveal detailed error messages to users; they can aid attackers.
• In testing, practice in a safe lab environment and study both how injections work and how to defend against them.

If you’d like, I can show concrete code examples in a language of your choice (Python, Java, Node.js, etc.) to illustrate safe vs unsafe patterns.