-1' OR 5*5=25 --

That looks like a classic SQL injection payload.

What it means

• The part -1' tries to close a preceding string literal in a SQL query.
• OR 5*5=25 is a tautology (true condition) that makes the WHERE clause always true.
• The -- at the end starts a SQL comment, so the rest of the query is ignored.
• Overall, if an input is unsafely concatenated into a SQL query, this can bypass authentication checks and return data the user shouldn’t see.

Why this matters

• It exploits unsafe string building in SQL queries.
• It can lead to bypassing login, leaking data, or manipulating the database.

Defensive strategies (exam-safe overview)

• Use parameterized queries (prepared statements) instead of concatenating user input.
  • Example idea: SELECT * FROM users WHERE username = ? AND password = ?
  • Bind username and password as parameters so they’re treated as data, not SQL code.
• Validate and constrain inputs:
  • For numeric fields, parse and validate as numbers.
  • For text fields, allow only expected characters, or use strict length checks.
• Use least-privilege database accounts for the app.
• Avoid building dynamic SQL strings from user input.
• Use ORM layers or safe query builders that handle escaping and binding.
• Consider input sanitization and a web application firewall for extra protection.
• Log suspicious inputs and monitor for injection patterns.

If you want, I can give a concrete before/after example in a language you’re studying (e.g., Python with sqlite3, Java with JDBC, PHP with PDO).