This is the process where you take data from each of the studies and store it in a data extraction form. You will have descriptive (e.g. type of study, study characteristics) and analytical data (e.g. outcomes).
Looking at other systematic reviews can give you an idea of the types of data to extract.
The Cochrane Handbook for Systematic Reviews is available online, and chapter 7.6 in Part 2 covers data extraction.
Additionally Cornell University Library have a comprehensive list of data extraction tools on their LibGuide.