Layman's terms, it's dummy data (or fake data, test data...etc.) that you can use to test or develop your code against until you (or the other party) is ready to present/receive real data. It's a programmer's "Lorem Ipsum".
Employee database not ready? Make up a simple one with Jane Doe, John Doe...etc. API not ready? Make up a fake one by creating a static .json file containing fake data.