It conveys a sense of intense focus and interest. It's a polite way of saying "I'm listening intently."
Here are some examples of how to use "all ears":
* "Go on, tell me all about it. I'm all ears."
* "I'm so glad you called. I was all ears when I heard your voice on the phone."
* "I'm all ears for any suggestions you might have."
The expression is often used in informal conversation, but it can also be used in more formal situations.