User gets a pop-up box to enter a memo every time while entering time in Time Card. There is set rule that makes time entry memos mandatory in Global Settings or at the project level.
You can control whether to show or hide the memo in the Time Card screen by clicking More > Show/Hide Entry Details. If you have chosen to show entry details, the memo box pops up every time you make an entry, but it is not mandatory to enter the memo if the corresponding project rule that makes memos mandatory is not in force. Check the Core Help for details. You just have to close it every time.