It's the last two letters you should be interested in.
"IR" stands for "Increment Repeat" and "DR" stands for "Decrement Repeat."

How the repeat works is rather interesting, actually.
For example, LDIR acts almost exactly like LDI, in fact, this code acts almost exactly the same (except for speed )
Effectively the pc does not change unless bc is zero; the instruction is executed over and over.
ldir_loop:
ldi
jp pe,ldir_loop