Preemptive multitasking - Running several processes/threads on a single processor, creating the illusion that they run concurrently when actually each is allocated small multiplexed time slices to run in. A process is "preempted" when it is scheduled out of execution and waits for the next time slice to run in.
A preemptive kernel is one that can be interrupted in the middle of executing code - for instance in response for a system call - to do other things and run other threads, possibly those that are not in the kernel.
The main advantage of a preemptive kernel is that sys-calls do not block the entire system. If a sys-call takes a long time to finish then it doesn't mean the kernel can't do anything else in this time.
The main disadvantage is that this introduces more complexity to the kernel code, having to handle more end-cases, perform more fine grained locking or use lock-less structures and algorithms.
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