Complex sentence: The source address is normally the address that the packet was sent from, but the sender address in the header can be altered to a different address, so that an attacker can make it appear that the packet was sent from a different machine.
Split into simpler sentences: The source address is normally the address that the packet was sent from. By forging the header so it contains a different address, an attacker can make it appear that the packet was sent by a different machine.