Spoofing is a trick played on servers to fool the target computer into thinking that it is receiving data from a source other than you. This in turn basically means to send data to a remote host so that it believes that the data is coming from a computer whose IP address is something other than yours. Let's take an example to make it clear:
Let your IP Address be: 203.45.98.01 (REAL)
Let the IP Address of the Victim computer be: 188.8.131.52 (VICTIM)
Let the IP Address of the system you want data to be sent from: 184.108.40.206 (FAKE)
Normally sitting on the computer whose IP is REAL, the datagrams you send to VICTIM will appear to have come from REAL. Now consider a situation in which you want to send data packets to VICTIM and make him believe that they came from a computer whose IP is FAKE i.e.220.127.116.11. This is when you perform IP Spoofing.