Idiom: Chasing rainbows
Example 1: Sarah is so beautiful and talented! I sometimes dream about what life would be like if she were my girlfriend. I'm too shy to talk to her, but even if I could, I know I would just be chasing rainbows. There's no way she would be interested in me.
Example 2: My friend is always trying new "get rich quick" schemes. She spends a lot of her money on lottery tickets and workshops that promise to help people achieve wealth in a short time. I try to help her be realistic and tell her that she is just chasing rainbows. Some people can get lucky and win the lottery, but most people won't.
Meaning: A rainbow is a beautiful arc of color that appears after the rain. A rainbow cannot be touched or held in your hands because it is merely a visual phenomenon created by the sunlight's effect on water droplets in the air. Therefore, when someone chases rainbows, this person is chasing/going after something that is impossible or unrealistic.
As in example 1, "Sarah" is the rainbow. The speaker knows it's an impossible goal.
In example 2, "getting rich quick" is the rainbow. It is an unrealistic goal.