Default Calibrator is an extension which appears in versions of the Mac OS from Mac OS 8.5 through to Mac OS 9.2.2.
Starting with Mac OS 8.5, Apple monitors got more and more professional: these monitors can be calibrated, i.e. set to exact color settings and other color-related variables. The DigitalColor Meter application is responsible for this.
This extension adjusts the user's monitor at system startup so that, color-wise, everything's perfect.