A good camera calibration will have a Final Total Error of less than 1.0 and very strong calibrations can have values under 0.4. Note this depends on the input marking precisions (Preferences / Process). This value is shown in the Total Error Dialog during processing or in the Project Status Report after processing. High error may be due to referencing errors, marking imprecision, a calibration grid that changes shape (i.e. between shots, perhaps due to it not being firmly affixed to the ground or other flat surface), or the calibration is over-parameterized.
In Single Sheet calibrations the automatic referencing error might occur with cameras with high levels of lens distortion - try increasing the number of photos each point must be referenced on (See Camera Calibrator section in Preferences - Processing and Cameras). Imprecision by the marker may be due to undue sharpening by a digital camera (turn it to softest setting if possible), or color artifacts from the digital camera (try the camera’s monochrome/black-white mode if it has one). A grid that changes shape can be remedied by retaking the photographs of a rigid and static grid. See The Calibration processing stage fails. for more information on over-parameterization.