Collected dirt/dust that the WD cleans out.
And that is, indeed, one of the things it is good for.
I had a car with a sticky seatbelt latch. OK when the weather was really dry, but bad when it was damp. Which was a lot of the time, 30 minutes south of Seattle, WA. Sniffing it told me it was some kind of cola spilled in it, probably multiple times.
So I hosed it out with 409 (water based cleaner/degreaser) on stream to wash it out, then WD40 to rinse out all the water. No oil added. Worked great.