The Columbian Exchange was the exchange of many kinds of plants, animals, foods, and microbes that happened when the Europeans reached the New World. It is called “Columbian” because Columbus was the first European to set up a permanent European presence in the New World.
The New World and the Old World were completely separate from one another for a very long time. This meant that different animals, plants, and microbes existed in the two land masses. It was only with Columbus that these organisms from the two land masses started to be exchanged.
There are some very famous aspects of the Columbian Exchange. Tomatoes and potatoes were brought from the New World back to the Old. Horses and cattle and sheep came from the Old World to the New World. Perhaps the most important thing that happened was that Old World microbes got to the New World. The people of the New World had no resistance to these microbes. This caused terrible pandemics among the natives of the New World.
Thus, the Columbian Exchange was simply what happened when the Europeans reached the Americas on a permanent basis and allowed plants, animals, and microbes to pass between the two land masses.