What is the general meaning of soliloquies, and what impact do they have on a reader?

A soliloquy is a dramatic device usually used in plays meant to be performed on stage. The term comes from two Latin words (solo and loquor) that mean "I speak to myself." It is a speech that a character delivers when no other characters are present on stage; it reveals the character's inner thoughts. Unlike novels, performed dramas do not allow the audience to read a character's thoughts. For audience members to know what a character is thinking, he or she must reveal it through spoken words or actions. Playwrights use the soliloquy to help audience members understand who a character is and to gauge a character's growth and change throughout the play. Sometimes a soliloquy can be used to explain details of the plot that can't easily be shown on stage.
One familiar soliloquy is Juliet's "Romeo! Romeo!" speech. Juliet stands alone on her balcony and speaks into the night. If she were to just stand on the balcony gazing into the darkness, viewers might suspect she is thinking of Romeo, but they wouldn't know for sure and wouldn't know, specifically, what she was contemplating. So Shakespeare allows Juliet to bare her heart, expressing her distress at learning that Romeo is from an enemy clan and professing her willingness to ignore that feud in order to be with him.
Soliloquies can have several different effects on viewers. At a minimum, they can explain the actions of the plot or attitudes of a character. Beyond that, they can make a character more sympathetic, as viewers are made privy to the deep emotions that drive him or her. It can work the other way, too: villains can soliloquize and reveal their twisted souls, causing viewers to despise them even more. Soliloquies often introduce dramatic irony. They let the audience in on secrets that other characters in the play don't know. This can increase the suspense of coming scenes, as viewers can anticipate actions before the characters in the play become aware of them.
Although the soliloquy is a dramatic technique that has become less prominent since Elizabethan times, it is still evident in the modern musical drama. The main character will usually have a song that she sings near the beginning of the play or movie that reveals her innermost desires. The Little Mermaid's "Part of Your World" and Moana's "How Far I'll Go" are two examples. These introductory songs, like their predecessor soliloquies, put the audience on the side of the hero and prepare the audience for the quest that will provide the plot structure.
https://literarydevices.net/soliloquy/