rain / reign are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
rain: (noun) water falling in drops from vapor condensed in the atmosphere. (noun) drops of fresh water that fall as precipitation from clouds. (noun) anything happening rapidly or in quick successive. (verb) precipitate as rain.
reign: (noun) a period during which something or somebody is dominant or powerful. (noun) the period during which a monarch is sovereign. (noun) royal authority; the dominion of a monarch. (verb) have sovereign power.
rain: r·EY·n
reign: r·EY·n
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)