At a guess, it'd mean building large slabs of concrete, or tanks of water, or other cheap item with large thermal inertia. Ideally you'd cool these down overnight, or at a period with cheap power, then under regular external temperatures you'd use these to reduce the need for cooling at periods of high power cost. Or, when it's very warm outside, these would work in tandem with the cooling systems to cope with a hot day, with the hope being that overnight you'd catch up, without needing to interrupt workloads.
