The legal term "tort" is defined as a wrongful act that causes injury to another. A mass tort, therefore, is a wrong, usually committed by a large corporation, that injures many people.
When a company commits a "mass tort," those who are injured may be able to sue the company and recover compensation.
For instance, assume a pharmaceutical company releases a defective drug into the marketplace, and that drug is sold to millions of Americans. Patients who were injured as a result of the defect – say, a faulty design – may be able to sue the pharmaceutical company for medical bills related to their treatment, as well as any other losses (e.g. lost wages, physical pain) stemming from their injuries.