Okay kiddo, so there are some people who call others "greasers" and they mean it in a mean way. It's not a nice word to say to someone.
A long time ago, in the 1950s, there were some kids who liked to wear their hair in a certain way and wear certain kinds of clothes. They were called "greasers" because they used a kind of hair gel that made their hair look shiny and slicked back, like they greased it up.
But, some people didn't like the way these kids looked, and they started using "greaser" as a way to make fun of them or put them down. It was like saying, "you look stupid and I don't like you because of the way you look."
It's not okay to use words like "greaser" to make someone feel bad. We should be kind to everyone, no matter how they look or what they like to wear.