The meaning of rein depends upon its context. For example:
"keep a tight rein on the new employee," or "rein in a horse" — to control or restrain
"give the new employee free rein," or "give the horse full rein" — do not restrain
"the reins of government" — means of control
"the reins of the horse" — leather straps used with a bit to control a horse
—hi!" and the clown cracking jokes behind him; and by and by all hands dropped the reins, and every lady put her knuckles on her hips and every gentleman folded his arms, and then how the horses did lean over and hump themselves!
Chapter 22 (57% in)
There are no more uses of "rein" in The Adventures of Huckleberry Finn.