Valid orders are orders that have been passed to Ometria from your ecommerce store as 'completed' - i.e. payment has been received, with no refunds processed.
This figure can change over time if orders are refunded at a later date, and the label 'valid' always reflects the data your ecommerce store sends Ometria via the API, and which order statuses you assign as valid.
E.g. some retailers choose not to count orders as valid until they are shipped.