A spod is a large normally rocket shaped tube with an open end that you fasten to the end of your line.
You then fill it with your chosen bait as freebies & cast it out like you would with a ledger or swim feeder.
when it hits the water it "up-ends" emptying the freebies directly over your swim/baited hook.
Hence the term "spodding"= using a spod to feed your swim.
You can now buy special "spod rods" that are designed for use with the spod feeder, these rods normally have a T/C of 4+lbs
the main aim of the spod is for those who do long range carping on the larger lakes (30 acres+) where its very hard (near impossible) to catapult or throw (with or without a throwing stick) the freebies in a concerntrated area.
Hope this explains it a bit.
I personally wouldn't bother with a spod because i don't fish any waters that are big enough to need one.