MacRumors:

Apple’s iOS 14 update adds several new features to the Photos app, including an option to add captions to images right from the iPhone, which is something that ‌Photos‌ users have been wanting for quite some time.

Tap to view any photo full screen on the ‌iPhone‌, and then swipe up to get to the caption field. Tap the field, type the caption, and press enter.

Captions in ‌iOS 14‌ sync across all devices if iCloud Photo Library is enabled, and the Description field in macOS Big Sur has been renamed to Captions for continuity.

There have always been kludges or other apps to accomplish this but it’s finally becoming a native feature.