Dimensions is a descriptive data point in the data that represents discrete values.
For example, What is the increase in cost of a product for each month? Here the month is a Dimensions and the increase in cost which is numbers are measures.
QlikView automatically classifies columns into Dimensions and Measures based on the type of the field.
If you are interested to learn QlikView, you can enroll in this course on QlikView Certification by Intellipaat.
You can watch this video on QlikView Tutorial to learn about Dimensions in QlikView: