What is the mean of JWT_DEFAULT_REALM?
The realm is part of the HTTP standard and send with an authentication request from the server to the client together with an authentication type (JWT in this case).
It describes the resource that is protected. In Basic authentication type it will be the resource name shown in the login box above the form fields. 
In authentication with JWTs it makes less sense since you would not expect the user to manually enter the JWT and browsers don't have special handling for the type JWT. So it will at max be included in an error message.
I am confused about the mean of JWT_DEFAULT_REALM or the mean of "The default realm" when I am using Flask_JWT. I am not a native English speaker.