Every year, voluntary turnover costs U.S. businesses roughly $1 trillion, according to Gallup. That is not a rounding error. It is the productivity, knowledge, and momentum that walk out the door with every resignation letter. Yet plenty of HR teams still treat retention as a feel-good idea rather than a number on the balance sheet.

Here is a better way to see it. Employee retention is one of the few people-management levers you can actually measure, forecast and improve on purpose. Largely by focusing on the everyday employee experience. Once you understand what retention is, why it matters, and which numbers to watch, you can spot risk early and act before your best people start refreshing their resumes.