Help! What does "replacing your motherboard" mean??

Dell has told me that they are sending someone out to 'replace my motherboard'. What does that mean? guess you can tell that I do ot understand computerese at all.
I have been provided by DELL a XPS M1530 laptop about 3 weeks ago to replace my 2 yr old XPS that they lost in shipping. They were taking so long to replace it that I called a woman whose name was in an article about the local DELL plant layoffs. She apparently pulled some strings, because within 36 hrs this "refurbished" computer was delivered to my door.
Personally, I think they grabbed one and quickly 'shot' it out to me....but....I don't think it had been repaired or whatever it is that "refurbished" means.
When I went to set it up, DELL showed it registered to some man......that call took nearly 2 hours.
AND.... the sot in th front for media cards ("8 in 1 media card reader") was missing the 'dummy plug' that protects it when not in use for downloading. Add to that my serious difficulty at getting my camera media card to fit 'just right' whenever I have downloaded any pictures, and then yesterday when I finally got the card in, the machine made it's little "da-dum" sound, but never recognized the card.....nothing ever came up on the screed. It wouldn't even work when I tried using the cable to transfer the pictures.

Can Anybody out there help me to understand what is going on . I'm distressed, depressed feeling hoodwinked and hijinked over this entire deal.

Thanks for any feedback that might clear , even a little of this muddy water for me. ^_^
Sheri