The Operator Symbol is a simple glyph consisting of a circle with an X crossing through it. It can alternately be typed as (X).
The symbol first appeared in Marble Hornets. Its true meaning is unknown. It is postulated by some, like M of The Tutorial that the symbol acts as a ward, keeping Slender Man at bay. However, other sources believe it only serves to draw him near. It is also often seen in the journals of those who have been stalked by him for some time, as in Just Another Fool. In this last occurence it may be a sign that the person drawing it is marked by him and doesn't have much time left.