The Expected Revenue field is an opportunity's Opportunity Value field multiplied by its Probability. For example, if the Opportunity Value is $10,000 and the probability of winning that opportunity is 50%, then the Expected Revenue is $5,000.
This field allows you to build forecast reports that reflect what's called the weighted value, weighted revenue or weighted pipeline.
You'll see Expected Revenue as a field option when building reports for opportunities.
You can add the field to any opportunity report, and even use it as a summary field. Of course, both the Opportunity Value and the Probability fields must contain numerical values for the Expected Revenue to display a number.