A "job role" is a description of what a person does. A "job title" is a convenient name for a role.

For example: At my last job, my role was to write code that helped us be sure that our product worked as well as we thought it did. My title was Software Engineer, because it's (as noted in the comments on your post) short and to the point. People know what a Software Engineer does in general, without a long explanation.