A soda jerk was a person who worked at a soda fountain. A soda fountain was a place where people could go to get ice cream and drinks, like soda.
The soda jerk was the person who made the drinks. They would use a big machine to mix together flavors, like cherry or vanilla, with carbonated water to make soda. They would then scoop ice cream into a glass and pour the soda over it.
The soda jerk would sometimes add things like whipped cream or a cherry on top. They also would sometimes add medicine to the soda to help people who were sick.
Soda jerks were really popular in the 1920s and 1930s. People would go to soda fountains to hang out with friends and have a sweet treat. Today, soda fountains aren't as popular, but you can still find them in some restaurants and ice cream shops.