This phrase "you must show, not tell" means that there was a concept of movies that the story must be told to the audience through actions and videos instead of the author telling the story through verbal voiceovers or dramatizations for the audience to understand.
The screenplay should be so well written that the audience must not need any summarisation o verbal entry, instead, they understand and enjoy what they have been shown.
The trend to change can take several years or decades about the choice of audience.
