A labeler can work on a task in the workspace. Read the documentation about the label mode for more details. The tasks for the logged in user are available within the route
/tasks. This route is available through the "Tasks" menu entry in the "Home" section in the navbar. The following image displays such an overview as an example.
By clicking the "Start Labeling" button one of the open tasks in that project is assigned to the currently logged in user until the task is skipped or finished. The workspace loads the task and the connected image can be labeled.
How many open tasks are available within one project is managed by the projects admin in the Tasks tab of the project.