The front-end and back-end portion of an application.

A full stack developer works on both the front-end and back-end portion of an application but also knows various other technology areas like Database, Infrastructure, Version Control, Project Management etc. What type of skills set is required to become a successful full stack developer?

Soft skills (personal attributes, traits and abilities.)

Communication
Attention to details.
Teamwork
Creative thinking
Adaptability
Time management, organizational and project management skills.
Problem solving skills
Critical thinking

Hard skills (skills that are required for the job, expertise necessary to execute the tasks in the role).

Front End Development (HTML, CSS3, JavaScript, React/Bootstrap/SASS etc.)
Back End Development (PHP, Database, other programming language)
Version Control (Git/GitHub)
Frameworks (Laravel, Django etc.)
Stack (LAMP/MEAN/MERN)

Experience at job

Hands on experience as front end and back end developer.
Delivery of few real projects
Working in teams

(Ref: Wikipedia, Stackoverflow and Google)