A to-do item can have the following dates or descriptions applied to it, to help you manage your day:
A due date is the day which the item is due. You can change this date by selecting the to-do item options menu and selecting "change due date".
Status refers to the progression towards the completion of the item — it can be Planned, In progress, Waiting, Completed or any of your chosen workflow statuses.
Last activity shows the last time the item was updated.
I am assigned a note with a to-do date of June 20th (for all intents and purposes, 20 days from now), so it appears on my to-do list in the Later period.
The to-do date is June 20th, but I might decide I need to start working on it sooner. So I drag the item to This week and am asked to choose a day this week, which will become the item's new to-do date. I choose June 3rd.
I then add a reply to the note to get help from a colleague. This will update the last activity part of the note, but will have no impact on all other timeframes.
When I mark this item as complete, the due date will remain, the item will appear in Completed, it will show up as complete (with a timestamp), and the last activity will be the day I checked it off.