Discuss the theory of seafloor spreading. 1 sentence or two

The theory of seafloor spreading is that new ground is created in the depths of the oceans, pushing older material towards the edges of the tectonic plates.  The ocean floors are then moving, from the center to the edges, dragging the continents along with them in the process.

Explanation:

This new ground/land is created along the mountain chains in the middle of the oceans floor as magma escapes from the mantle of the planet and when cooled down by the ocean's water creates a new oceanic crust.

This theory created in the 1960s is quite largely accepted now. It explains lots of details of the continental drift and the friction of the tectonic plates against each other.