UWSA stands for UWorld Self-Assessment. These are timed practice exams from UWorld, a commercial question bank company, that many examinees take in the weeks before a USMLE exam to check readiness and pace. They are not produced by the USMLE program, NBME or ECFMG.
How a UWSA result relates to a real score is a matter of opinion, and examinees disagree about it. Treat the number as one data point, and compare it with NBME self-assessments, which come from the organization that writes the exam. The USMLE program does not publish any official conversion from practice results to real scores.
See NBME self-assessments for how to use practice exams alongside each other, and when to take each Step before you book.