This is the 6387th most frequent Chinese word.
护城河
Moat.
Here, '护城河' refers to a physical moat built around ancient cities for defensive purposes.
古代城市周围常常有护城河。
Ancient cities often had moats around them.
In this sentence, '护城河' metaphorically refers to a competitive advantage or differentiating factor that protects the company from competitors.
企业的优质服务是它的重要护城河。
The company's high-quality service is its important moat.
'护城河' here is used literally to discuss the history and development of moats as structures.