Here's a breakdown:

* Mid-ocean ridges: These are underwater mountain ranges that run through the middle of the ocean basins. They are where new oceanic crust is created.

* Process: Magma from the Earth's mantle rises up at the ridge and cools, forming new oceanic crust. This new crust is pushed away from the ridge on either side, creating a process of "spreading".

* Evidence: The pattern of magnetic striping on the ocean floor, which is consistent with the magnetic reversals recorded in the Earth's history, is strong evidence for seafloor spreading.

So, to sum it up: Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is generated and then pushed away from the ridge, causing the ocean floor to spread.