A running user is defined by the settings, it can be either the logged in user, or a specific user. Specific user is for individual specific users (not necessarily yourself). So a specific user can be a running user, but a running user is not necessarily a specific user. Does that make sense? Here's a link to some documentation: https://help.salesforce.com/articleView?id=dashboards_select_running_user.htm&language=en&type=0
A Running user can be defined as user who is currently login into salesforce.specific user means can be defined as user who is there in the salesforce it can be running user or other users in the salesforce
https://help.salesforce.com/articleView?id=dashboards_select_running_user.htm&language=en&type=0
A Running user can be defined as user who is currently login into salesforce.specific user means can be defined as user who is there in the salesforce it can be running user or other users in the salesforce