Dashboard running user and dynamic dashboards
Key Takeaway
A dashboard shows viewers data through its running user's access. Setting "View Dashboard As" to "The dashboard viewer" makes it a dynamic dashboard, so each viewer sees data based on their own access.
Every Salesforce dashboard runs as a running user: viewers see data through that user's access to records. A dynamic dashboard runs as whoever is logged in, so one dashboard can show each person only the data they are allowed to see. You then don't need to build a separate dashboard and folder for each person.
How the running user decides what viewers see
A static dashboard shows data using one configured user's privileges. With this fixed running user, every viewer sees the same slice of data.
For example, imagine a running user who can see opportunities only in Canada. The viewer sees only Canadian opportunities. If the running user can see opportunities from any country, viewers see all countries.
A dynamic dashboard instead shows data based on the logged-in user.
Because the running user's privileges decide what displays, the running user can give viewers access to more data than they normally have. For example, if the running user is an admin or an internal user, viewers could see personally identifiable information (PII).