MAC spoofing basically means that another device on the internet could have the same MAC address as your network card or IP device. All network devices have a unique MAC address which is a hex value. The MAC address is then bound to the network protocol, be it IP, IPX etc. MAC addresses are used as the final stage of network address translastion.

If somone has spoofed your MAC address it basically means that they could be receiving your network traffic instead of yourself. MAC addresses are cached on your local machine against any other network devices you have accessed.
To see a list of IP/MAC addresses stored on your local machine go to a command prompt and type the following: arp -a
this will then show you a list of IP address against their physical MAC address. MAC addresses in theory can only exist once and cannot be changed at the hardware level as it is set by the device manufacturer. Hackers today have of course overcome these obstacles by using MAC spoofing etc.