Body composition describes what the body is made of: lean muscle, fat tissue, bone and water. Two people with identical weight can have very different compositions, which is why scales and BMI tell only part of the story.
Tracking composition shows whether weight change is coming from fat or muscle, which matters because losing muscle lowers metabolic rate and worsens rebound risk. DEXA scanning gives the most accurate picture, and doctors use it to set baselines and verify quality of progress.