What is or what "fast-ramping batteries" means at this context?
"Ramping" up or down is a utility grid term for how quickly a generator can add power to the grid, or reduce its output when demand reduces. 

Steam generating plant cannot ramp fast - it can take hours, up to days for some nuclear plants to reach full power again after a shutdown. Other plant - especially hydro - can ramp up as fast as you can turn a tap.

There's nothing stopping battery storage reaching full power instantly, which makes it "fast ramping" -valuable for stabilising a grid in the presence of variable load - or sudden changes in generating capacity if a plant shuts down.

This allows better - and cheaper - frequency control.