There was an unspoken understanding between them that no words were needed.

unspoken 是由动词 speak 的过去分词 spoken 加上否定前缀 un- 构成的形容词。un- 表示"不、未"，spoken 表示"说出来的"，所以 unspoken 意思是"未说出口的、不言而喻的"。 understanding 在这里指"理解、共识"，unspoken understanding 表示"无需言明的默契"。