An adjustable waist compression band for the midsection, worn under clothing or during training.
What it actually does
It compresses the waist and supports the lower back while you wear it. It makes a shirt sit flatter and takes some load off the back during lifting or long periods standing.
What it does not do: it does not burn fat, it does not permanently reshape your waist, and it does not do anything once it is off. Anyone selling it on that basis is not being straight with you.
Fit
Adjustable closure so you can set the tension. Measure your waist and check the size chart rather than guessing.
A caution
Do not wear it so tight that it restricts your breathing, and do not sleep in it. If you have a back injury or hernia, ask your doctor before using abdominal compression.