All 4 Uses

rein
in
The Fellowship of the Ring
(Auto-generated)

Strider sprang from hiding and dashed down towards the Road, leaping with a cry through the heather; but even before he had moved or called, the rider had reined in his horse and halted, looking up towards the thicket where they stood.†
Chpt 1.12

reined in = restrained
At that moment the rider sat up, and shook the reins.†
Chpt 1.3 *
'Now then!' said the farmer, throwing the reins to Sam and striding forward.†
Chpt 1.4
He reined his horse in, and halted, swaying in his saddle.†
Chpt 1.12

Definitions:

(1) (rein as in: to rein in their activities)
to restrain or control; or a means of control
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 (You might like to think of other senses of rein as being figurative derivations from this sense.)
(2) (meaning too rare to warrant focus)
to "give free rein" is not to restrain (not to exercise control over)