HEIC explained in two minutes
Maybe you emailed an iPhone photo and the reply was "I can't open this." That was HEIC. Here is the short version of what it is and why it causes trouble.
What HEIC is
HEIC is a container that uses the HEVC video codec. It stores a still photo with the same compression used for 4K video. A HEIC photo takes roughly half the space of a JPG that looks the same.
Apple made HEIC the iPhone default in 2017. The goal was to save storage without making photos look worse.
What a HEIC file can contain
People say HEIC to mean an image stored in the High Efficiency Image File container. That container can hold more than one image. It can also hold thumbnails, transparency, depth data and metadata. So a basic previewer and a full photo library app may read the same file in different ways.
The extension doesn't tell you which extras are inside. Most iPhone photos are plain still images with metadata. Portrait and burst shots can carry extra image data. A basic JPG export flattens that data or leaves it out.
Why HEIC won't open everywhere
HEVC is covered by patents and is fairly new. Because of that, a lot of software outside Apple's ecosystem never added support. Older Windows versions, web upload forms and some photo editors don't recognize the format.
The photo isn't broken. The app on the other end can't read HEIC.
Which format to convert HEIC to
JPG is the safest choice for photos you send to forms, email, older software or print shops. PNG helps when you need transparency or exact pixels. For a normal photo, though, PNG makes a much bigger file and adds no detail.
If the place you upload to supports modern web formats, WebP can be smaller than JPG. When you convert for compatibility, JPG is still the reliable default. Check which file types the site accepts before you convert a large batch.
The fix
Convert a copy to JPG when you need to share it widely. Keep the HEIC original for yourself. JPG is lossy, so it can't hold every detail of the source. With the original saved, you have a clean archive and can make a different export later.
Start with a high JPG quality setting. Check fine hair, leaves, text and smooth gradients at full size. For public sharing, remove location data if you don't need it. In your private archive, keep dates and camera details if you find them useful.
Avoid converting the same photo twice
Each time you open and save a JPG again, it can pick up more compression artifacts. Make every new copy from the HEIC original, not from an older JPG. The damage shows most along sharp edges and in textured areas.
Use clear filenames or a separate export folder. Then you can tell the original from the copies at a glance. This small habit stops a compressed copy from becoming your master file by mistake.
Before you convert a whole batch, test a few photos first. Include a portrait mode shot, a screenshot, an edited photo and a very large file. Check orientation, color, size and metadata. Then use the same settings for the rest of the library.