Let’s start from the beginning! The definition of holding (or flying a hold) in aviation field means some manoeuvres of aircraft before the landing. Incoming aircraft must sometimes be stacked when an aircraft which has arrived at their destination but cannot land yet because of the traffic congestion, poor weather, or runway unavailability.
Stacking means that pilots go into a holding pattern circling radio beacons.
Holding stacks work as waiting room for an aircraft before landing. Then the aircraft is in the “waiting room” the air traffic controller efficiently organises the landing directions and runway. For this reason, there can be several aircraft separated vertically by 1,000 feet or more at the same time in holding stack. When the plane leaves the holding stack it is safely directed towards the runway.
One airport may have several holding stacks; depending on where aircraft arrive from or which runway is in use, or because of vertical airspace limitations. For example, around Heathrow airport there are four holding stacks called Bovingdon, Lambourne, Ockham and Biggin.