This symbol is the Mabuni family crest (mon), dating back many centuries. It was adopted by Kenwa Mabuni to symbolize the Karate system he founded and developed, Shito Ryu Karate Do.
The circle represents "peace" and "harmony" (wa), while the lines inside the circle represent the Japanese character for the word "human" or "person" (hito). So, the emblem can be interpreted as "people working in peace and harmony".
Kenwa Mabuni also used the symbolism of the two lines to represent his two main teachers, Itosu and Higaonna.