The theory of plate tectonics states that the Earth's outer shell is made up of several large, rigid plates that move and interact with each other. These plates can move away from each other, towards each other, or past each other.
The theory of seafloor spreading is a specific aspect of plate tectonics that explains how new oceanic crust is created at mid-ocean ridges, where plates move away from each other. As the plates move apart, magma rises up from the mantle and cools, forming new crust on the seafloor. Over time, this process can create a long chain of underwater mountains, or mid-ocean ridge.
The theory of plate tectonics supports the theory of seafloor spreading because it provides a mechanism for how the plates move and interact with each other, which ultimately leads to the creation of new oceanic crust at mid-ocean ridges. In other words, plate tectonics provides a framework for understanding how the Earth's outer shell behaves and how the continents and oceans have evolved over time.