Here you can specify which users will be able to manage products in the current department. To add a user, click the Add users button and check the boxes next to the appropriate users in the displayed selection dialog. Users can be removed from the list at any time using the corresponding checkboxes together with the Remove selected button. You can also remove all listed items at once by clicking the icon and performing the Remove all action.
More resources concerning users can be found in Developer's Guide -> Development -> Membership, permissions and security.
Further information about departments can be found in E-commerce Guide -> Managing your store -> Departments.