IP spoofing means to change the source IP of a packet to be other then the source IP of the host.
Doing so is usually to conduct DOS/DDOS attacks.
IP spoofing is usually used in UDP and ICMP packets since they are stateless protocols.
IP spoofing is hard to detect, unless the spoofer uses unallowed addresses (like 127.0.0.1, 192.168.x.x, or the local subnet of the target), so don't be fooled to companies' statements that can detect spoofed packets, because there's no way to do so.
TCP spoofing is a bit different and is a source for alot of confusion.
Spoofing is demonstrated in the sample PacketCrafter.