Before I begin, I want you to know the difference between a monologue and a soliloquy. Hecate's speech is a monologue.
A soliloquy is where a speaker talks aloud regarding their own thoughts.
A monologue is a long speech to someon else.
While they seem very similar, I assure you that Hecate delivers a monologue given she is not reflecting on her own personal thoughts, but speaking to the three witches.
Hecate is seen as the supreme witch in Macbeth. It is during her monologue in Act III, Scene iv, where her power becomes known. While the witches have shown Macbeth their prophecy, and it has come true, Hecate knows even more. She knows that Macbeth will return to ask the witches about his future with the crown.
The theme conveyed in Hecate's monologue is one of fate. In the closing lines of the soliloquy, Hecate states:
His hopes 'bove wisdom, grace and fear:/ And you all know security/ Is mortals' chietest enemy. (III,V,76-78)
Here, Hecate speaks of the fact that a man secure in his future is one who does not fear. She knows that Macbeth will find a false security in the three apparitions prophecies. She also knows that his security is false.
That being said, Hecate's monologue illustrates the theme of fate that is intertwined throughout Macbeth. She, only, knows the true fate of what is to come. Macbeth has seen what he believes to be his fate, or destiny-as provided by the witches' first prophecy. Unfortunately, fate., itself, is proved to be far more superior than the trust in fate alone.