![1girl censored greyscale hat heart heart_of_string komeiji_koishi kurona monochrome panties panties_around_one_leg photoshop_(medium) pussy sitting solo spread_pussy touhou underwear rating:e score:5 post #368894](/cdn_image/preview/dc/86/dc8638c249a6156a7fbd16a32b177a66.jpg)
Pictures with a line that forms a heart-shaped loop. It's not really important if it's supposed to be an actual string, it may be a ribbon, a typographical element or a background decoration, etc.
The equivalent Pixiv tag is ラインのハート, literally "heart made of a line".
Komeiji Satori and her sister Komeiji Koishi sport these frequently.
This tag implicates heart (learn more).