Once you have completed the screening of your studies, you will have identified the ones which will be included in your review. But you will still need to determine whether the studies are reliable. In Chapter 4 of Doing a Systematic Review: A Student's Guide, the term 'quality' is described as "The degree to which a study employs measures to minimize bias and error in its design, conduct and analysis" (p. 65).
There are numerous tools for critical appraisal available - choose one which is appropriate to the types of studies you are considering in your systematic review. A useful list of systematic review examples and the quality assessment strategies they used is in Ch. 7 of Systematic Approaches to a Successful Literature Review (2nd edition).