What does it mean to have a mild personality disorder?

Not sure.: Everyone has a personality or traits that characterize their behavior. When the personality or traits becomes so "unusual" as to cause enduring and significant personal or social distress it crosses the line between being "normal" and being a disorder. I suppose mild means you are approaching that border.

Personality disorder: The key distinction between a personality disorder and other mental disorders is the degree to which the maladaptive coping patterns are longstanding & interrelated to the individual's sense of self. Qualifiers like "mild" typically have little evidentiary basis, rather they qualitatively describe the number, severity or pervasiveness of symptoms. Licensed clinician with specialty is key to tx.