To track additional information for each time entry on the Timesheet you can either use Comments field or Custom Fields (Timesheet Level Custom Fields or Entry Level Custom Fields, which help in tracking specific predefined data). Entry Level Custom Fields can be set for each entry in the Timesheet.
The Comments field is used to get custom information about each time entry and has limitations when tracking through reports, as they cannot be part of calculations. They are merely used for saving additional information related to the time entry. On the other hand Custom Fields can be used in report data calculations as well.