Bealzebubs excellent analogy:
It is a virtual reality version of the lobster/fish tank in a posh restaurant.
Candidates who might be deemed suitable for employment are told that they will be considered again as future vacancies arise. Sometimes those that are removed from employment will be told that they will be considered again if and when new opportunities arise. It gives the company a resource to possibly draw from at short notice rather than having to re-initiate a new recruitment programme.
The pool or tank in a busy restaurant is a useful feature that lets the customer select their own choice of product, with the knowledge that it looks fresh and attractive. In a busy restaurant the pool can be added to as necessary, ensuring a ready source of supply.
Inevitably, sometimes the product will become damaged or cease to be fresh in the pool and it can then be removed, discarded and replaced with something fresher, or if the market contracts can be emptied altogether. The pool/tank is a resource for the restaurant. Not every fish or lobster will become a fine culinary presentation. The best will usually be selected first, the leftovers will be discarded or cycled into other products as their longevity and lack of freshness make them less appealing to the customer. Fresh additions will often be selected ahead of older survivors.
When the market for fresh lobster drops off, the product can be humanely thrown back in the sea. The tank can be emptied and cleaned with bleach. It may provide window dressing for a while, but if it is taking up space and/or costing money to run, it is likely to be put in a cupboard.
An airline holding pool has many of the same characteristics. It provides a resource for the company. It can pick and choose who it might want from that pool at its own whim. It can add to it at any time. It can empty the pool or discard items as it wishes. Unlike the real pool, in this virtual pool the product is only "an expression of interest" that might have moved on elsewhere. It costs little or nothing to maintain, and because it is virtual, it provides no guarantees of priority of supply, for either the candidate or the company.
Airline "holding pools" are therefore simply a group of people who have both given and received an "expression of interest," at a particular point in time. There is no particular commitment on either party. As time passes some of the denizens of the holding pool will cease to be as attractive to the company that interviewed them. Likewise some might have found employment elsewhere. A "holding pool" is not a job, or even a promise of a job. The companies expect the candidates to realise this and act accordingly. The candidates themselves should realize this and accept that as a product they have a definite shelf life in this respect.