Eligibility criteria are the inclusion and exclusion rules that defined the study population. They are the first thing to compare against your patient, because they determine whose result the trial actually reports. A patient who would have been excluded sits outside the evidence, and applying the result to them is an extrapolation rather than a direct application.
A trial that excluded the patient in front of you
A trial of a new anticoagulant for atrial fibrillation enrolls adults under 75 with normal kidney function and excludes anyone with a prior stroke or a creatinine clearance below 50 mL/min. Your patient is 81, has had a stroke, and has a creatinine clearance of 38 mL/min. The trial's relative risk reduction of 30% was measured in a population that does not include her. You can still consider the drug, but the expected benefit and the bleeding risk in her case are extrapolations from a lower-risk group, and the trial cannot tell you how large they are.
Exclusions are often reported in a flow diagram rather than the abstract. The number and reasons for exclusions after enrollment also matter, because a trial that screened 5,000 patients to enroll 200 describes a narrower population than the enrollment number alone suggests.