IP fragments are a certain type of IP packets that are not sent at once but in multiple parts. The destination or target system has to reassemble the pieces into an IP packet. There are legitimate reasons why fragmentation can (and must) occur. One example of the legitimate uses of IP fragments is for a router that connects networks with different MTU's. It has no choice but to create IP fragments (eg. FDDI -> ethernet transition). Excessive fragmenting however, could be a serious warning you have a problem.
There is a detailed paper on this issue available:
We expect an increase in attacks using IP fragments as more of these tools become available to the (would be) hacker community.