rung of a ladder
Refers to the horizontal crosspiece of a ladder. The word is straightforward and literal.
One rung of the ladder is broken.
I climb by placing my feet on the rungs of the ladder.
Compound of 梯子 (はしご, 'ladder') + の (possessive particle) + 段 (だん, 'step' or 'tier'). The reading is straightforward and the meaning is transparent.