No one answered this yet? Everyone was watching IRL then. ;-)
"When computer is in use" means when you are busy behind the keyboard, moving your mouse about. Similarly, "when computer is idle" is when you're not busy behind the keyboard, moving the mouse about.
These options work together with the "Suspend work while computer is in use?" option. setting the preference to 1% when the computer is not in use will mean that BOINC is only allowed to use 5MB on your P4 2.4, 10MB on your P4 3.2 etc. Not enough to run Seti with.
You can then just as well use the "Suspend work if no mouse/keyboard activity in last x minutes" option, which will suspend all calculations after so many minutes of no keyboard/mouse activity.
Ancient Astronaut Theorists suggest that in many ways, you can be considered an alien conspiracy!