Breast cancer is one of the most prevalent malignant tumors in women worldwide. Nearly half of patients exhibit a human epidermal growth factor receptor 2 (HER2)-low expression profile, rendering them ineligible for traditional HER2-directed therapy as they fall under the category of HER2-negative breast cancer. The distinct clinicopathologic features of HER2-low breast cancer as compared with HER2-negative breast cancer have intrigued a growing number of researchers. This review provides a comprehensive overview of advancements in the definition of HER2-low breast cancer, its characteristics in epidemiology, clinicopathology and prognosis, its molecular genetic features, heterogeneity, and relevant agents. Emerging evidence suggests that HER2-low breast cancer may represent a distinct clinicopathological subtype. However, current data remain insufficient to establish it as an independent prognostic factor. Furthermore, the lack of standardized criteria for interpreting HER2 immunohistochemistry (IHC) 0 and 1+scores has led to inconsistencies in classification. The absence of refined subclassification within the HER2 IHC 0 population may further obscure potential benefits from targeted treatments for this group. This review emphasizes the need for future studies to refine and standardize the diagnostic procedures and treatment regimens for HER2-low breast cancer. Establishing consistent diagnostic protocols and treatment strategies may enhance clinical management for these patients.