I haven't read the text, but data drift refers to how, after deploying a machine learning model, the input data changes over time to something that wasn't tested on. For instance, let's say you create a gradient boosting forecasting model that does a great job at predicting tomorrow's earnings. At the time of training, the earnings might be in the $1000 per day range. But a year later, the earnings might be in the $100k range. The model has never seen numbers this high before, so it doesn't know how to handle them well. That is data drift.