What does George Herbert Mead mean by learning to "take the role of the other?"
George Herbert Mead thought that a person's personality develops through social interaction. This can be contrasted with Freud who placed a larger emphasis on biology.
The individual mind exists only in relation to other minds and this is via shared meanings. Mead presents the self and mind as functions of social processes. Mead thought that existence in a society comes before individual consciousness. That is, one can't become self-conscious until one interacts in a society and takes the perspective/role (attitude) of others. This social interaction precedes the formation of the individual.
For example, I can't think of myself as a self until I encounter other selves and learn how to react to them as they react to me. I take in the attitudes of others (roles, gestures, signs, reactions, communications, behaviors) and then react with those roles and other attitudes I've learned in social interactions.
Mead speaks of this individual-social interaction in terms of "I" and "me." I'll use Karl as a case study. Karl's "me" is his collected attitudes of others, what gestures, behaviors, and signs he picks up from others. Karl's "me" becomes how Karl thinks others perceive him. Karl's "I" is his individual reaction to those accumulated attitudes, his "me." The "I" is the subject, the part of Karl that acts. The "me" is the object, how Karl thinks others see him. Karl must take the role of the other (collection of attitudes); in a sense, this is like saying Karl must take on the language of the other in order for communication to be possible.
Mead thought that children first take on the role of a significant other (a parent or sibling), someone whose opinions matter, who has influence, or who means the most to them. And this is often a kind of mimicry like when a child acts like his mother. The child sees his mother as an individual; therefore, the child thinks that to be an individual, I must act like mother. Again, the child develops his sense of self through social interaction with others. In this case, where the child acts like his parent, he is directly "taking on the role of another."