Notices

I installed nitdroid, my files where there on the SD card, and I did these magic commands:
(from here: http://www.youtube.com/watch?v=WY839T78qcY)

root
cd /home/user/MyDocs
bzip2 -d gingerbread_superuser.tar.bz2
cd /
mount /home /and
cd /and
tar xvf /home/user/MyDocs/gingerbread_superuser.tar

Cuz I wanted to overclock nitdroid, now there is not one file on my SD, it appears as a 5GB in windows explorer but it is a *GB SD card, did I delete my sd card or just unmounted it?
If so how do I go back?
Will the overclocking be affected?

No, you didn't erase anything. Actually you didn't do anything with your SD card.
According to the commands you provided and asuming they all were successful you now should have the contents of the archive in: /and

But there are two things that might have gone wrong:
1. Do you have a mount point called /ant? By default you don't. So if you didn't create it this didn't work. I that case you should have seen an error message here about a missing directory. As a consequence the subsequent cd command didn't work either and the contents of the archive should be directly in / now.

2. Regardless of the outcome of 1. you most likely don't have enough space in / or /ant (you don't have it on an unmodified N900). So you should have received an error message about insufficient disk space.

The worst thing that might have happened is that your / or /home partition is full now.

btw:
There's nothing "magic" about these commands. These are basic unix shell commands. I'd suggest to get familiar with them and understand what they are doing first before you try any further "magic".

translation so that the magic disappear:

get admin rights
Change Directory to /home/user/MyDocs
use bzip2 with option Decompres on the file gingerbread_superuser.tar.bz2
Change Directory to /
attach filesystem from /home folder in the /and folder
Change Directory to /and
use tar with options eXtract, Verbose, File on the file /home/user/MyDocs/gingerbread_superuser.tar

Well I understood the code, exept for some parts like xvf. The magic comes when they mix together
I actually did not receive any warnings in the terminal, witch puzzles me. So how would I go and remount my sd card? Thanks for your help