It means that you are filing single (not married or head of household) and also claiming one person as a dependent. This dependent could be a qualifying child or relative.
The IRS offers these 5 tax filing statuses:
- Single
- Married filing jointly
- Married filing separately
- Head of household
- Qualifying widow(er)
You may qualify for more than one tax filing status. For example, a single (unmarried) person isn't necessarily restricted to the single filing status. He or she may be able to file as head of household or qualifying widow(er), assuming those qualifications are met. But if he or she wasn't legally married as of 12/31/2017, they couldn't file their 2017 return as a married person.
From: https://ttlc.intuit.com/replies/3302320