These are the skills that you're generally unable to convey in your résumé, but are the characteristics that should come across in an interview. Not only can you make an impression with your soft skills, but those qualities are what your colleagues and supervisors will call upon when you use them as references for a new job. These are what make you stand out from a pack of candidates who have only developed strong hard skills. Those are the technical aspects of a job, and while absolutely important and necessary to get the job done well, they don't point to the deeper value you can add to a company.
Unlike hard skills, soft skills are thought of as difficult to measure and observe, and include your people skills, attitude, the way you take criticism, communicating, conflict resolution, delegating, planning, etc. You can't be trained to learn soft skills like you can be trained to develop hard skills, but you can always be aware of your strengths and challenge yourself to work on your weaknesses.