A kong is four identical tiles. In American play you expose it on the rack.
Some card lines ask for four of a kind. Hong Kong and MCR also kong, with replacement draws and scoring. American play follows the card.
A joker may finish a kong.
Four tiles, then a replacement
A kong is four identical tiles. After an American call you expose them and usually draw a replacement. A quint is not a kong; it is a different shape on the card.
A joker can finish a kong. Chinese tables score concealed and melded kongs differently. American play reads whether the line wants four, and whether they are exposed.