Soy Boy is a term that describes men who don't adhere to traditional gender norms and typically act feminine. However, they don't feel a disconnect from the gender they were assigned at birth (assuming they are cis). Trans men can also be soy boys; in that case they still present or act quite feminine, but feel no disconnect from the male gender.
Soy contains a form of estrogen and it's thought that consuming large amounts of it (often in the form of soy milk or tofu) can feminize a man.
The term is usually used in a derogatory manner. It's commonly used as an insult by the alt-right for more left-leaning individuals, especially men who support LGBT+ rights, feminism, or veganism. Some men try to reclaim the term or use it ironically.
Soy Boys in the Media
Internet Comment Etiquette, a famous Youtuber known for off-color humor, often calls himself a soy boy (sometimes while drinking soy milk and/or soy sauce, sometimes while dousing himself with it) in response to people insulting him in the comments.