Simplicity is a noun that means "the quality of being plain." The simplicity of your prom dress made you stand out in the sea of ruffles and sequins.
Simplicity describes a lack of ornamentation, like a Christmas tree that is only decorated with white lights instead of being fancied up with lots of ornaments, tinsel and multi-colored bulbs. The noun simplicity also means "freedom from hardship or effort" like the simplicity of hanging out on your porch. Simplicity also describes something easy to accomplish or understand.