The Erythrocyte Sedimentation Rate (ESR) test measures how quickly red blood cells (erythrocytes) settle at the bottom of a test tube over an hour. A higher-than-normal ESR indicates inflammation in the body, which may be due to conditions like infections, autoimmune diseases, or cancers. While the test doesn't pinpoint a specific disease, it helps assess the presence and intensity of inflammation. It's commonly used to monitor diseases like arthritis or other inflammatory conditions.