A fundamental requirement of the National Labor Relations Act (NLRA) is that employers must not "interfere with, restrain or coerce employees in the exercise of their rights" to organize into labor unions, collectively bargain and engage in similar "concerted activities," according to § 8(a)(1) of the law.
In general, "concerted activities" occur when at least two employees take actions intended to improve their wages or working conditions. It can also mean action taken by one employee, such as communication with a supervisor. What that actually means in practical terms is spelled out on a case-by-case basis.
The U.S. Court of Appeals for the District of Columbia Circuit recently did just that in the case of Hyundai America Shipping Agency, Inc. v. National Labor Relations Board (No. 11-1351). The litigation began when a terminated employee, Sandra McCullough, complained to the National Labor Relations Board (NLRB) that she'd been fired for engaging in "protected concerted activities."