When a tacked up horse is left in his loose box either tied or untied it is usual to twist the reins together and fasten them through the throatlash. This prevents them from slipping over the horse's head and getting tangled in his legs resulting in broken reins and/or a panicked horse.
To release the reins undo the throatlash at the arrowed buckle and allow the reins to untwist. When refastening the throatlash it should be done up with enough room to allow 4 fingers to be slipped between it and the horse's cheek.
You can similarly check the noseband - you should be able to insert 2 fingers.
Taken From New Rider