This field allows you to connect the column or measure you wish to show as the main value to which other indicators (Comparison Value, Target, Target for Variance and States) are related. Connecting a measure or column to this field is essential to display bars. If a Category is connected, the field will only accept one measure, displaying one bar for each category value.

Or, it can accept more than one connected measure when the Category field is not populate.

Read how the visual works with more Value measure at Multiple Measures.