pancake的意思是“薄煎饼”,它的特点就是又平又薄，和这种煎饼一样平坦，as flat as a pancake可以引申为“非常平坦、一马平川”。

【例句】
I like going climbing. That's why I dislike areas that are as flat as a pancake.
我喜欢登山，因此我不喜欢一马平川的地方。

Most of our county is as flat as a pancake.
我们县大部分地方非常平坦。