How can the theory of plate tectonics explain how the Atlantic ocean is growing by a couple centimeters a year?
The Mid-Atlantic Ridge is a divergent boundary.
The Mid-Atlantic Ridge is located, as the name suggests, in the "middle" of the Atlantic Ocean. That is to say that it is located approximately halfway from the eastern United States and Africa, as shown below (please note that the U.S. and Africa are used as references only, because there are many countries that border the Mid-Atlantic Ridge).
It is classified as a divergent plate boundary. This means that magma effusively erupts at this point, creating a new lithosphere that is spreading away from each other in opposite directions (see image 2 below). As this occurs, it forces the lithosphere at the continental plates to be pushed away from their current position. The result is a widening ocean. In general, this is a very slow process, but something that can be measured in our lifetime.