Tasks in Marvin are specific action steps you can complete in one work session.
A typical task in Marvin is about 5-60min long.
A couple examples of tasks:
Email Jane about dinner
Send off finished report
Share blog post on twitter
Read article "XYZ"
Do Math problems 3.4 and 3.5
Edit article for 40min
If there is something you need to do that takes you several hours or days to complete it is best to set it up as a project instead.