A self-deprecating person knows her own weaknesses and shortcomings and isn't afraid to point them out, often in a humorous way.
Some people are egotistical, always talking about how great they are. The opposite type of person is self-deprecating, dwelling on his own faults and even joking about how he's short, unsuccessful, or not that smart. Being self-deprecating is usually considered a good trait, a quality of someone with a wry sense of humor. When being self-deprecating goes too far, it can become self-loathing and self-sabotaging, which are less amusing forms of putting yourself down.