hi is the time spent processing hardware interrupts. Hardware interrupts are generated by hardware devices (network cards, keyboard controller, external timer, hardware senors. ) when they need to signal something to the CPU (data has arrived for example).
Since these can happen very frequently, and since they essentially block the current CPU while they are running, kernel hardware interrupt handlers are written to be as fast and simple as possible.
If long or complex processing needs to be done, these tasks are deferred using a mechanism call softirqs. These are scheduled independently, can run on any CPU, can even run concurrently (non of that is true of hardware interrupt handlers).
The part about hard IRQs blocking the current CPU, and the part about softirqs being able to run anywhere are not exactly correct, there can be limitations, and some hard IRQs can interrupt others.
(As an example, a "data received" hardware interrupt
from a network card could simply store the information "card ethX needs to be serviced" somewhere and schedule a softirq. The softirq would be the thing that triggers the actual packet routing.)
si represents the time spent in these softirqs.
A good read about the softirq mechanism (with a bit of history too) is Matthew Wilcox's I'll Do It Later: Softirqs, Tasklets, Bottom Halves, Task Queues, Work Queues and Timers (PDF, 64k).
st. "steal time", is only relevant in virtualized environments. It represents time when the real CPU was not available to the current virtual machine - it was "stolen" from that VM by the hypervisor (either to run another VM, or for its own needs).
The CPU time accounting document from IBM has more information about steal time, and CPU accounting in virtualized environments. (It's aimed at zSeries type hardware, but the general idea is the same for most platforms.)