Plain means mostly informal
Guests may sport a “plain” outfit but must wear at least two pieces of "fancy" apparel with their "plain" outfit. Remember, no "all plain" outfits are allowed!

Need some suggestions?

Fancy...
for women:
anything that sparkles, ball gown, beaded handbag, cashmere sweater, cocktail dress, diamond brooch, evening gown, feather boa, high heels, pashmina scarf, rhinestone choker, scarves, stoles, tiara

for men:
ascot, button-down oxford, cashmere sweater, cummerbund, derby, diamond cufflinks, fedora, formal wear, Rolex watch, satin handkerchief, silk tie, suit, tuxedo, wingtips

Or mostly plain (with a touch of fancy)...
baseball cap, boots, bowling shoes, capri-length pants,
casual attire, cowboy hat, football jersey, hiking boots,
jeans or denim, loafers, mules, slingbacks or flats, polo shirt, slippers, sport coat, sweatsuit, tennis shoes, trousers, V-neck t-shirt

PLUS must add two items from the “fancy” list!