A timer service is a scheduled task management service installed on every SharePoint server in the farm.
As a single unified logical entity, a SharePoint farm requires a mechanism to run tasks necessary to provide its services. These tasks include updating components of the farm such as servers and services, and updating data and configuration in farm databases. To run these tasks, SharePoint provides its own scheduled tasks management service, manifested as Timer Service instances installed on every SharePoint server in the farm. (-Microsoft)