EWS means economically backward section. EWS rank is the rank among the EWS students only. For example, if 1000 candidates attended the exam and there are 100 EWS candidates; if you are better than 500 candidates overall and better than 50 candidates from EWS then your general rank is 499 and your EWS rank is 49.

Further Reading:

• EWS Reservation Eligibility
• UPSC Age Limit