Drift, or axis creep, happens on the X-axis (or "yaw" axis) with earlier MPU6050-based devices because the sensor cannot accurately measure movement around a vertical axis. The sensor relies on gravity applying 1G of force onto the axes to provide a known reference point. However, the x-axis pivots around a vertical - imagine a line straight down through the top of your head, around which your head spins. This is in perfect line with gravity, so gravity does not apply across that axis when you're looking dead-ahead - gravity runs straight down the axis line.
This prevents the sensor from being able to 100% accurately measure rotation when in that position, but it uses an internal process of axis fusion to estimate it. It does a pretty good job, maintaining dead-ahead for normally 30 minutes or more - but it can go off to one side over time... this is the drift that people speak of. Typically you should not be seeing drift all that regularly - you should be able to play for at least half an hour before needing to re-centre the device. Accurate calibration is the key to minimising drift.
The EDTracker Pro and MPU-9150 versions of the device do not suffer from yaw drift when calibrated correctly.