Robot fetish is a variety of sexual fetishism in which a person becomes sexually aroused by descriptions or depictions of robots, usually ones in erotic situations or positions.
Some examples of fetishes under the broader category of robot fetish include erotic electrostimulation, fetishes involving wires, both varieties of temperature play and just about any others you can think of. Since robot body parts can typically be swapped out and the cultural ideas of robots are so diverse, almost any kink can be played out in the context of robots, though not all would necessarily fall under the category of a robot fetish.
It's some fans of the Transformers franchise are sexually attracted to the robots (usually when they're in their "root" or more humanoid form). Sex between transformers can be depicted in a variety of ways, many of which fall into the category of robot fetishism, though transformers fic writers can and do use the Transformers easily modified and decidedly non-human bodies to play out any kink or fetish one's heart (or other body part) desires. The transformers fandom has made it abundantly clear that even stereotypically organic kinks like breeding, heat/rut, pregnancy, nursing, feeding, and aphrodisiacs can all be done with robots.
Of particular note in the transformers fandom, is that it's rare for a character to have only "male" or "female" equivalent sexual organs, but rather most are depicted have both or neither (in which case sex or "interfacing", as its commonly called, is achieved using other means). This may be because Transformers media has not historically balanced the species gender demographics males or male-equivalent robots (who use male pronouns are are coded male) are significantly more common than female or female-equivalent robots. (It should also be noted that Transformers has a large and long lived queer fan base, though cis/het fans are also common.)
Detroit: Become Human:
The most common ship in the Detroit: Become Human fandom is between the robot/android, Connor, and his human police partner, Hank Anderson. Sex and sexual situations around Connor are diverse and may play with the idea of his lack of or possession of human equivalent in various ways.