Mind Units (MU) constitute the principal scoring mechanism in Ingress. It is a measure of the human population that is under the influence of a control field.
When a control field is created (or destroyed), the scanner displays the total number of mind units captured by (or liberated from) that control field. The scores you see on the intel map or the “Intel” tab of the ops screen are expressed in mind units. Note that the same mind units can be captured multiple times by layering control fields.