A more thorough explanation:

Protected activity refers to conduct that is allowed or encouraged by a law or the constitution, and for which the person cannot legally be punished or retaliated against.

For instance, the Civil Rights Act's Title VII prohibits an employer from punishing an employee who opposes a discriminatory employment practice or assists in investigating an allegedly discriminatory employment practice. If an employee is punished for engaging in any of these activities, they can sue their employer.

Another example of protected activity is whistleblowing. If an employee reports illegal or unethical behavior by their employer, they are protected from retaliation under various whistleblower protection laws.

These examples illustrate that certain actions are protected by law, and employers cannot punish or retaliate against employees for engaging in them.