A skill set is a combination of abilities, qualities and experiences you can apply to perform tasks well. These can include soft skills such as interpersonal skills, organization and leadership as well as technical skills such as research, computer programming, accounting writing and more.
Spending time on improving your skills can help you achieve personal career goals such as earning a promotion or becoming an expert on a certain topic. Your skill set can be applied to progress in your current career or expanded to earn a job in a different field or industry.
An assessment of your skills is a good place to start understanding any gaps that might need filling in order to reach your goals.