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. Note that MU amounts are based on estimated population and won’t be completely accurate; you may, for example, capture surprising amounts of MU from fielding bodies of water. The scores you see on the intel map are expressed in mind units. Note that the same mind units can be captured multiple times by layering control fields.
Capturing MU helps you progress towards the Illuminator medal.