Hi. the best way for me to describe this is when a train company recruits they get thousands of applicants. lets say they only had vacancies for 20 people, if 50 people passes all the stages pf interviews, psychometric testing and Multi-modal interview, the company will most like fill the vacancies with the 20 they need (sometimes in order of scores) and the other 30 will be basically on a waiting list. You can be on this waiting list for however long the train company needs until they have people who have left through retirement, for other jobs etc... they will then contact people in the talent pool and fill those gaps. This also saves them recruiting on a too regular basis.
Companies recruit for talent pools based on their expected requirements for staffing in a set year. There’s usually far more people in talent pools than there are jobs unfortunately. It’s in the company’s interest I guess, but not ideal for candidates. You often find that you’re in a pool indefinitely and the TOC/FOC then recruits for the same role again, making you wonder what the point is.