A mid-ocean ridge is an underwater mountain range. This mountain range is formed underwater between 2 oceanic plates at a divergent boundary.
Each year these plate boundaries pulled apart from each other causing magma to rise from the mantle to fill the gap at the plate boundary.
The magma cools and solidifies on the surface forming part of the crust. The plates hence diverge away from each other causing solidified magma to move away from the plate boundary as well. This is what causes ocean spreading.
This discharging and solidification of magma at the plate boundary forms a mid-ocean ridge over time which is a series of submerged mountain range.
Some examples of Mid-ocean ridges are: