The FBI issued a warning against using public charging stations in places like airports, hotels, and shopping centers.
They say that bad guys have figured out ways to use public USB ports to "introduce malware and monitoring software onto devices."
So you could be unknowingly connecting your phone to bad stuff through the USB, which includes software that can give hackers access to your device.
There's a simple work-around though: Just bring your full charging cable WITH the brick. There aren't any issues with charging through a plug, it's just when you stick your cable directly into a USB port.
The FBI didn't specify any specific locations or cases, they're just calling this a general advisory.