Your computer communicates with the devices attached to it through IRQs (interrupt requests). When an interrupt comes from a device, the operating system pauses what it was doing and starts addressing that interrupt.
In some situations IRQs come very very fast one after the other and the operating system cannot finish servicing one before another one arrives. This can happen when a high speed network card receives a very large number of packets in a short time frame.
Because the operating system cannot handle IRQs as they arrive (because they arrive too fast one after the other), the operating system queues them for later processing by a special internal process named
ksoftirqd is taking more than a tiny percentage of CPU time, this indicates the machine is under heavy interrupt load.