GB11's answer summarized it well what are the tracking devices. From the YouTube page of GPSSports Systems you can see that what Zlatan wears is called GPSports SPI HPU tracking device, which according to its brochure has many features, some of them being GPS, accelerometer, wireless communication, water resistance, 5.5 hours battery life, etc.
The data collected by tracking device from players is auto-analysed to provide practical information on players' performance, like:
- Establish the session load for each player
- Identify outliers in the group
- Identify players under significant training stress
- Compare sessions to recent history
- Produce high quality reports
- Periodise training using a range of variables
- Monitor individual training demands
- Monitor rehabilitation and reconditioning programs
- Compare players, sessions, drills and games
Note that those kind of tracking devices are used on a very high level in many team sports, and not just in football.