This quote was by Sir Albert Einstein, one of the best quotes in the genre of memorization. As beautiful it sounds it has this exceptional usefulness and meaning in each of our lives.
The pure meaning is that one should avoid memorizing pure data that is already available in books or sheets. Pure data is something that will eat up your time in memorizing and is waste of efforts.
This mostly applies to students in the science field. More importantly is the understanding of the concepts, theories, formulas, etc… What I think Einstein referred to is utilization of the knowledge and concepts that you’ve learnt so when there is a need for it again, you’ll know where to find the reference materials. Without the background knowledge, what you will be reading will seem like a foreign language.
So basically learn the concept and logic and not memorize data.