Secure Store Service Concept in SharePoint 2013

Why Secure Store Service Application in SharePoint ?

When data is stored in external source (anything other than SharePoint content database data is considered to be external ), SharePoint has to make separate request to display external data on SharePoint pages.

How this is achievable? This can be achievable with SharePoint Secure store service, it's important to understand that the username and password used to access external data stores may not be the same as the username and password used to access SharePoint.                                                                                                                                                                                                                    

When we are requesting data from external source from SharePoint, that user should be known to external data system, only then SP will be able to display the data.

The Secure Store Service is designed to create a background mapping between a group of users in SharePoint, and a single user known to the external data system

Note : Whatever the account (It is the single user account mentioned in above point ), that you have mentioned under set credentials section that should be given PSDATAACCESS on the respective external database.