The person in the image is wearing a GPS sports vest, often referred to as a "GPS bra". These are used in professional sports to track player performance data during training and matches.
The vest holds a small GPS device in a back pocket to track movement and collect data.
Data collected includes distance covered, speed, heart rate (with a monitor like the one visible), and other metrics.
Coaches and analysts use this information to optimize training, manage player workload, and prevent injuries.
They are made from comfortable, breathable, and often neoprene material designed for athletic activity.