The term "sted-son" seems to be a misspelling or a variation of the English phrase "step-son." In adult content, it often refers to situations where a character is the step-son (or child) of one or both characters involved in the scene. This could lead to various sexual scenarios and power dynamics depending on the context of the video.
A typical scenario involving a "sted-son" could be a stepfather having an intimate relationship with his younger stepson, exploring taboo desires and breaking societal norms. This theme is often found in categories such as incest or family roleplay within the adult industry.