The usually unnamed simple aesthetic, called the "s tag aesthetic" by tumblr user kinoko-teikoku, usually features white backgrounds and clean, simple photographs. There are also images from drawr or pixiv. Color schemes with a simple aesthetic are usually fairly simple -- monochrome or dull colors, focusing on a light, clean feeling.
Images of food, especially eggs, are somehow common. Photos of food products whose labels bear simple, colourful designs on a white background are also often seen.
The full concept of s tag music has not been visualized yet, but it most likely is any song that has a major sound, and features a light sound, without using too many instruments. A good example would be songs by the Korean Bossanova duet Casker.
Usually any text involved (whether in lyric form or on photos) are devoid of much meaning, or are "simple" in themselves, and are a little melancholy or happy. The tone is not overly emotional on the whole; emotions remain unexpressed or are subtle.
Simple aesthetic fashion often features white sneakers and round glasses.
Dresses and shirts often feature patterns on a white background, or a single, simple object printed in the middle. A lot of dresses, shirts, and socks also have a graph pattern.
Pants are loose and long; navy jumpers or jumper skirts are often in use.
Hairstyles are loose and often braided.
Photos of Korean and Japanese fashion models are often seen wearing this sort of style.
Stationery is another frequently seen aspect of the simple aesthetic -- especially photos of Japanese, Chinese or Korean stationery with white backgrounds.
Pantone is a (probably unexpected) part of it.