A greaser is a type of subculture that became popular in the 1950s. A subculture is a group of people who have their own way of dressing, music they like, and things they do that are different from the rest of society.
Greaser culture originated in America and was characterized by men who had greasy hair that was slicked back with pomade, wore leather jackets, tight jeans, and white t-shirts. They loved fast cars, dancing, and rock and roll music.
The name "greaser" came from the fact that they used a lot of hair grease to style their hair. They were also sometimes associated with getting into fights, but mostly they were just rebellious and didn't want to follow society's rules.
To summarize, greasers are a group of people who love rock and roll music, fast cars, and dressing in a certain way. They have a distinct hairstyle and love to rebel against society's rules. They are called "greasers" because they use a lot of hair grease to style their hair.