raining / reigning / reining 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. 👇
raining: (verb) precipitate as rain. (adjective) falling in drops or as if falling like rain.
reigning: (verb) have sovereign power.
reining: (verb) stop or slow up one's horse or oneself by or as if by pulling the reins. (verb) stop or check by or as if by a pull at the reins.
raining: r·EY·n·ih·ng
reigning: r·EY·n·ih·ng
reining: r·EY·n·ih·ng
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)