The term Quality Assurance comprises of two words - "Quality" and "Assurance". In order to understand Quality Assurance, we first need to understand the meaning of the term 'Quality'.
Quality is the conformance of a system to its specified requirement or features. The quality of a software product is measured based on different attributes like - correctness, reliability, robustness, usability, completeness, efficiency etc.
ISTQB defines Quality Assurance as-
Part of quality management focused on providing confidence that quality requirements will be fulfilled.
Basically, Quality Assurance is a set of activities to prevent non-conformance of a system to its requirements. It ensures that the built software product meets all the specified requirement of the customer. Unlike Quality Control(QC), we don't actually test the software product in QA.