Why was Zhu Yuanzhang called the founding emperor?

First of all, we must understand what is the "founding emperor"

"Founding": As the name implies, it is to establish a new dynasty.

"Emperor": the supreme ruler of China in ancient times and every dynasty (starting from Qin Shihuang).

Therefore, the "founding emperor" is the supreme ruler who created a new dynasty.

Zhu Yuanzhang started a new dynasty-Ming Dynasty, and was the supreme ruler. So he is the "founding emperor".