If you get an email or a letter that officially announces you've been laid off from your job, you can call it a pink slip. Some hard workers get pink slips because their employers are trying to save money by eliminating positions.
In some cases, workers receive literal pink slips — notices (traditionally on pink paper) included with their paychecks informing them that they've been fired or laid off. Pink slip is mainly figurative these days: if you say, "I got my pink slip today," you mean "I lost my job today." The term probably started in vaudeville theater at the beginning of the 20th century, when stage acts received cancellation notices on pink slips of paper.