Further details about hours can be entered into the Notes (optional) field when creating or editing a time entry from the Day view of your timesheet.

To view notes in a time report (Reports > Time) or project analysis page, click any of the hour totals in the report. You’ll be taken to a detailed time report, with each time entry listed, notes included. Notes will also appear on CSV and Excel exports of the detailed time report.

To include notes on invoices, you'll need to have each time entry displayed as its own line item. When creating an invoice, next to How to display hours, choose Detailed: Display each time entry as one line. When the Time entry details module pops up, be sure Notes is checked.