Talent pools are virtual communities of passive job seekers stored in a database which allow HR teams to pipeline future employees. Wikipedia provides a longer definition, but the idea is that talent pools allow the employer to build communities filled with candidates who are interested in working there in the future. It provides you with the ability to engage passive candidates frequently and to tap into this reserve whenever you need to streamline any hiring process.
Talent pools help both the job seeker and employer. As a job seeker, I might not be ready to apply yet.