It's weird - even AI singled out "g" when I looked up reign vs rain - see "g" reat
Rain refers to water falling from the sky (precipitation), while reign refers to the period a monarch rules or the act of ruling. They are homophones (sound the same) but have different spellings and meanings. A helpful tip: A king or queen has a "g"reat reign, and rain is water.
This video explains the difference in pronunciation and meaning between the words: