What HEIC actually is
HEIC is Apple's implementation of the HEIF format. It stores the same photo at roughly half the size of a JPEG with no visible loss, which is why iPhones adopted it. The trade-off is compatibility: many Windows apps, older programs and web upload forms still do not read it.
Option 1: change the iPhone setting
Go to Settings, then Camera, then Formats, and choose Most Compatible. New photos will be saved as JPEG. Existing HEIC photos stay as they are, and files sometimes still convert during transfer, which is why this alone is not always enough.
Option 2: convert the files you already have
For photos already sitting on your PC, converting to JPG is the quickest fix. A browser-based converter handles this without installing codecs or paid software, and the photo never leaves your computer.