A wave cut platform is made at a place where there is a cliff next to the sea. Destructive waves slowly erodes the cliff by corrasion and hydraulic power

The continuous erosion creates a wave cut notch which begins at the high sea level line and ends and the low sea level line.

As the erosion continues the wave cut notch becomes a cave. As now there is nothing to support the cliff face the force of gravity weakens the rock.

As a result from the constant force of gravity on the cliff face it breaks off from the cliff and is broken down by the sea by the rocks hitting each other (attrition). This leaves a wave cut platform from where the cliff face used to be.

Pictures from Google images