fetish
Explanation
The comic shows two time travelers in their time machine. One explains that if you go back in time and give one specific guy a weird fetish, then 20 years later everybody thinks they have that fetish for the next hundred years. The caption reads: "Time travelers love visiting Freud."
The joke is a satirical take on Sigmund Freud and his outsized influence on psychology. Freud famously theorized that many human behaviors and neuroses were rooted in sexual desires and fetishes (such as the Oedipus complex). The comic suggests that Freud's bizarre sexual theories weren't genuine insights into human nature -- instead, some mischievous time travelers went back and planted weird fetishes in Freud's mind as a prank. Because Freud was so influential, he then projected those fetishes onto all of humanity, and people believed him for over a century. It's a humorous explanation for why Freud's theories seem so strange by modern standards.