Great question — answering as the UphillTech team:
They are performance trackers. They track a player's vital statistics (heart rate, etc) as well as their movement, their average speed, their distance covered, etc. As for the "bra-like" design, it would appear to just be the result of needing a small and unencumbered design that allows for minimal interruption, maximum body contact, and full range of motion. Link to the manufacturer of Zlatan's bra
Full answer by GB11: https://sports.stackexchange.com/a/16710 (CC BY-SA 4.0).
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