This template is identical to Template:Flag, except that the flag links to its image description page, for pages where the flag is for information rather than just decoration, or for non-public domain images for which attribution is required.
The c in the name refers to "clickable".
Example
{{flagc|USA}}→ USA
Compare:
{{flag|USA}}→ USA - image is not clickable{{flag icon|USA}}→ - image links to the country concerned