To track work time by tasks in addition to projects use the project and taskstracking mode. You can set it on the Group settings page. Projects and tasks tracking mode in Group settings
To manage tasks use the Tasks tab in the menu. This capability is available to users having the manage_tasks access right (default Co-manager, Manager, and Top manager roles). Task list in Time Tracker
The Required checkbox on top of the Tasks page is responsible for making a task selection mandatory on time entry and edit pages. Use the Add button to create a new task. A screen like below appears. When adding a task you can select projects associated with it. Adding a new task in Time Tracker
You can use the Edit and Delete icons in the task table to manage already existing tasks.