Time entries you add to Time Logs can be either billable or, not billable.
The billable time entries are used in handling billing information and generating billing reports, while the other are not.
By default, all time entries added to Time Logs are marked billable. This is governed by the 'Billable' checkbox on the 'Add Time Log' tab, and by the 'Add new entries as billable' checkbox on the 'Weekly View' tab. If you do not want a time entry to be marked billable, remove either of the boxes, depending on which tab you are.
You can also unmark an existing time entry as billable at any time. To do so, use the procedure described in Editing time entries.