A due date is for an item that is actionable starting now, and has a definite time when it must be completed by. When you create a to-do and add a due date, the to-do goes onto your timeline right now to let you know that this is a current priority, but the exact timing of the work until its due date is up to you.
The due date for a to-do will also show on your calendar since your calendar is a place for items that have a concrete relationship with time – and your calendar items are also shown on the timeline. If you have items that are due today, they should show as calendar events at the bottom of the timeline.
A reminder is for an item that you don't want to begin working on yet. A reminder for a to-do will not show in your timeline until the time you set the reminder for. Please note that you cannot have a reminder for a to-do that is inside of a project - in projects, only due dates are allowed for both the project and the to-dos inside.