I actually needed to look that up, since I didn’t know either. So, thank you, I’ve learned something new because of your question.
d/s The number (after merges) of discard requests completed per second for the device. f/s The number (after merges) of flush requests completed per second for the device. This counts flush requests executed by disks. Flush requests are not tracked for partitions. Before being merged, flush operations are counted as writes.
man7.org/linux/man-pages/man1/iostat.1.html
(Discards being TRIM operations on SSDs, I imagine.)