At this point in your systematic review, you should have a pile of studies that appear to answer your research question. All of them are slightly different: different populations, different outcomes, different study designs, and even different results.
Before you can write a comprehensive report about this existing research, you need to transform those piles of studies into digestible and organized pieces of information.
You do that by going through every study and pulling out the details that will help you answer your question.
In other words, you extract your data. You might also see the phrase “data abstraction” in reference to data extraction. These terms are used interchangeably.