Secure Store Application ID
Each Secure Store Service entry contains an application ID that is used to retrieve a set of credentials from the secure store database. Each application ID can have permissions applied so that only specific users or groups can access the credentials that are stored for the application ID. Applications use application IDs to retrieve credentials from the secure store database on behalf of a user. The application can then use the retrieved credentials to access a data source.
I will walk you through - how to create Secure Store Application ID that you can use in External Content Type.
a) Go to SharePoint Central Admin then Manage Service Applications.
b) Click on Secure Store Service Application. If Secure Store Service is not setup, then you have to create one. Look at my article How to Create Secure Store Service.
c) Click on 'Generate New Key' on the ribbon.
d) Enter pass phrase as you like.
e) Now we will have to create Secure Store Application that would help in impersonation. Click New on the ribbon.
f) Enter the required values for the target application settings. Click Next.
f) You can add more fields if you like, for now use the default Windows Username and Windows Password. Click Next.
g) Specify Administrator for the target application. You can specify Members for the target application as well if you like to. Click Ok.
h) Now your Target Application has been created successfully.
i) Now tick the check box beside the Target Application then click on Set Credentials on the ribbon to set the credentials for impersonation.
j) Enter a credential owner, the windows username and windows password that will be used for impersonation by this target application then click Ok.
Now Application ID has been created we can use this target application for impersonation to access BCS External list.