Rein is a noun that refers to a strap or rope used to control or guide a horse or other animal. It can also be used as a verb to mean to control or guide with a rein. For example:

• He held the reins in his hands. (noun)
• She reined in the horse to stop it. (verb)

Reign is a noun that refers to the period of time during which a monarch or other ruler holds power. It can also be used as a verb to mean to rule or dominate. For example:

• The reign of the ancient Egyptian Pharaohs. (noun)
• He reigned over the kingdom for many years. (verb)

In summary, rein is a noun that refers to a strap or rope used to control or guide an animal, and a verb that means to control or guide with a rein, while reign is a noun that refers to the period of time during which a monarch or other ruler holds power, and a verb that means to rule or dominate.