-1' OR 5*5=25 --

This looks like a classic SQL injection payload.

What it does (high level):

• -1' ends a string literal in many SQL dialects (terminates the current string).
• OR introduces a logical disjunction.
• 5*5=25 is a boolean expression that evaluates to TRUE (since 25 = 25).
• -- starts a comment, so the rest of the original query is ignored.

Combined, if injected into a login-like SQL statement, the condition can become something like: WHERE ... OR 5*5=25 --, which is always true, effectively bypassing the intended checks.

Defensive takeaways (exam-ready):

• Use parameterized queries (prepared statements) instead of string concatenation. Example:
  • SELECT id FROM users WHERE username = ? AND password = ?
• Validate and constrain inputs, and rely on the database to compare values via parameters, not via string-building.
• Use least-privilege database accounts and proper error handling (don’t reveal DB errors to users).
• Prefer ORMs or query builders that automatically parameterize inputs.
• Consider additional protections: input length limits, WAFs, and proper error messages.

If you want, I can give you a quick practice question set on SQL injection and defenses or a brief worked example with parameterized queries.