no seriously, what is it?
blood type
Web definitions
blood group: human blood cells (usually just the red blood cells) that have the same antigens.
wordnetweb.princeton.edu/perl/webwn
edit:
In layman's terms, it's essential a different 'type' of blood you have, and this restricts who you can share blood with. If you had A, and you got B blood from Dodger, well you wouldn't be too well off.
Ask a health teacher, they can explain way better than I can. It's kinda their job to know.