Flashing in simple terms is something like re-installing windows.

You can find more about it here
wiki.maemo.org/Updating_the_tablet_firmware

You will need two things.
1. The OS file/image. Look up the link above, it has a link within for the N900 files
2. The Flasher program. Again, the link is in the wiki.

3*. The wiki will also talk about a eMMC file. But you can leave that for now

Basically you download the OS file. Install flasher software(it has no graphical interface and is run from the command prompt) and then use it to reflash the N900.

Hope it helps. Though, if you still have doubts after reading it. Ask again