Genesis 37 tells the story of Joseph and his brothers. Verse 19, "They said to one another, “Behold, this dreamer comes," is a sneering remark spoken by Joseph's brothers. It's important to understand the context to fully grasp the meaning:

Joseph's Dreams: Joseph had been sharing his dreams with his brothers, dreams in which they all bowed down to him (Genesis 37:5-11). These dreams, combined with their father Jacob's obvious favoritism towards Joseph, fueled their jealousy and hatred.
"This dreamer": The brothers use this term derisively. It's not a neutral observation; it's a sarcastic jab. They're mocking Joseph and dismissing his dreams as mere fantasy. They see him as arrogant and self-important for believing such things could come to pass.
Jealousy and Hatred: The brothers already resented Joseph because Jacob loved him more and had given him a special coat. The dreams were the last straw.
Foreshadowing: Ironically, while the brothers dismiss Joseph's dreams, the story ultimately reveals that these dreams are prophetic and will eventually come true.

So, when they say, "Behold, this dreamer comes," they're expressing a combination of:

Contempt: They look down on Joseph and his supposed delusions of grandeur.
Jealousy: They resent his perceived superiority and the implications of his dreams.
Anger: They are frustrated that their father favors him and that Joseph seems to think he is better than them.
Dismissal: They don't take his dreams seriously and believe they are impossible.
Intention: It is a declaration of animosity before plotting to harm Joseph.

In essence, the line is a cruel and mocking greeting, a prelude to the violence and betrayal that follows. It perfectly encapsulates the brothers' hatred and sets the stage for the dramatic events of the story.