Gravity batteries are a renewable energy storage mechanism, still being developed for the mainstream. In theory, they are very similar to pumped hydroelectric. In times of low-demand and cheaper electricity, water is pumped up a hill into a reservoir. When electricity demand is high, and price is high, they let the water fall through a generator using the force of gravity.
Gravity batteries are another take on this principle; store energy when it is cheap and plentiful and generate electricity when it is not - through using gravity.