Patrol deployment can be defined as the determination of what officers should be where and when they should be there. It is the process of managing the police force's manpower in an attempt to provide the best possible police coverage for the citizens of a jurisdiction.
Police departments need to figure out how to deploy their officers. They need to think about, for example, whether to have them in cars or on foot or on bicycles. They need to think about whether to have one officer per car or two officers. They need to also think about where to have officers patrolling and when. The point of all of this is to ensure that officers will be available to respond as quickly as possible to emergency calls while still getting something worthwhile accomplished in between calls.
Patrol deployment, then, is the process of figuring out the best ways to employ a police department's resources.