The number of unique device Ids recorded within a 30 day period is referred to as 'Unique Visitors'. If a device ID records multiple visits within the 30 day period, it is considered as the same unique ID/Visitor.
For example, If a person visits your website five times a week from the same device, they would still be counted as one unique visitor.
The number of 'Unique Visitors' across each product discovery channel (Search, Categories, Collections and product recommendations) is listed on the home page of the dashboard.