"One's primary skill" refers to the main or most important ability or talent that a person has. It is the particular expertise or competency that they excel at or are known for. Full definition
If you study the sample below you'll see a clear communication of primary skills. (resume-resource.com)
For people whose primary skill is strategy, it's all about organizing their thoughts, ideas, experiences, skills, expertise, and expectations to accomplish the desired goal. (realtormag.realtor.org)
However, those abilities are to be additional, not primary skills on your resume and / or cover letter because they are not a priority for available job openings today. (prime-resume.com)