Answer:
Moshi and her best friend have agreed on the meanings of each of the symbols.
Explanation:
A symbol is a thing that is used to represent something else, particularly when a physical object is used to represent an abstract thing.
Therefore, those phrases used by Moshi and her best friend are symbols they have jointly agreed upon to use as representations specific meanings. They are effective because of their joint agreement on them. Any other phrases that they have not jointly agreed upon will have no meaning to them and they will not be effective.