Who can tell me about this?

Who can tell me what's the meaning of "a state of mind."and how to use it's used in daily life?  Thank you!

The English dictionary says: "a temporary psychological state."

A "state of mind" refers to the current mood or emotion(s).  

For example:

If someone says "Sohma's state of mind is optimistic!" it means that I am feeling good or looking foward to a future event.

If someone says "Sohma's has a bad state of mind" it means I'm angry, upset, worried, or not looking foward to an upcoming event.