"What does it mean to be anti-speciesist?" It means not being prejudiced against other species.
"What is an example of being prejudiced against other species?" Thinking that it's not okay to breed, abuse, and murder humans for pleasure, but paying for other species to be bred, abused, and murdered by paying for non-human animal-based products such as bacon, beef, eggs, and milk for the sake of taste pleasure.

"Okay, but honestly this kind of sounds like ♥♥♥♥♥♥♥♥. Humans are different from animals. We're higher on the food chain. It's the natural order. Why should I give a ♥♥♥♥?"
Starting point: "Dominion" available for free on dominionmovement.com/watch OR "The Best Speech You Will Ever Hear" by Gary Yourofsky on Youtube at https://www.youtube.com/watch?v=U5hGQDLprA8&ab_channel=GaryYourofsky

"Okay, but we need to eat animal products to be healthy, right?"
Health: The Game Changers on Netflix.

And in case that's not enough, eating animal products also creates one of the major causes of environmental destruction and degradation.
Environment: Cowspiracy on Netflix.