You "yell at" someone when they've done something wrong and you're angry at them. In most cases, a person in a higher social position yells at the person in a lower position. For example:
Ms. MacGuire used to yell at me all the time for talking during class.
I got yelled at for not calling my parents.
The meaning of "yell at" is similar to the word "scold", but English speakers almost never use "scold" except in formal written English. "Yell at" is the common way to express this.