The Great Merchant is a Great Person acquired though Specialists or Wonders.
"Great Merchants" provide the goods and services civilizations need to survive. They import food, medicines, and luxuries, and they export a civilization's manufactured goods. They provide the financing needed for great works to be accomplished. They create a civilization's wealth.
Besides constructing the special Customs House improvement which, when worked, produces loads of Gold, the Great Merchant can also journey to a city-state and perform a "trade mission" which produces gobs of Gold and gains many points of Influence with the city-state. The Great Merchant is expended when used in either of these ways.
Again, you have the choice to use it for a long-term boost of the Gold production of one city (choose this city well), or use its unique ability to get a large one-time boost of Gold and Influence with a City-state. Note that the Trade mission may sometimes prove dangerous, especially if the City-state is far away.