How are plate tectonics and seafloor spreading related?
Seafloor spreading is plate tectonics.
Plate tectonics may seem like a relatively simple topic but can actually be very complex when taking into account all of the aspects of liquid and solid earth.
The reason we have plate tectonics is because of a viscous, partially molten liquid mantel or asthenosphere which lies just beneath the solid crust or lithosphere. The solid crust of the earth slides very slowly on top of the mantel. The crust is broken up into many plates which collide and shift around each other all the time.
How seafood spreading works into plate tectonics is that where the plates meet their can either be convergent or divergent plates. Sea floor spreading is just another word for divergent plates. When plates diverge from each other, underneath the ocean, new sea floor is created. When the plates diverge from each other, other plates are also converging which is what creates mountains and subduction zones. In the grand scheme of earths history, that's how the planet recycles rock and our atmosphere on a multi million year timescale.