Please read the below post from bhen the water drains around the door seals blah blah on how to properly post if you want proper help in the future, thanks.
As you didnt state anything about your car, I will assume that you have an S-series, which is known for this problem, which I again assume is that when water drains around your door seals (working as designed) the water collects under the snap-on door sill plate.
All S-series do this, it is a piss-poor design, but is not an imediate rust threat if you keep that area dry. A pain I know, but I run a thin towel or chamois under that plastic rocker panel cover to absorb any excess water. I have seen many s-series with rust in that area because it was not kept dry. Also, on SC2's keep the 3rd door latch dry, it will also rust.
On the 1st gen S-series, the snap on plate covers a gaping hole, harder to work with. On the 2nd gens, up to '02 models, the plate snaps into three small slots in the rocker panel. What sux is that this is also covering the weld seams on the rocker, so beat to keep it dry. As the cover isnt solid, no luck on permanently sealing it up for me, any ideas anyone? marx404