I wasn't prepared at all when my parents sat me down to tell me that my mother had been diagnosed with cancer, more specifically, a brain tumor.

sat 是动词 sit 的过去式，意思是"坐"。 down 是副词，表示"向下"的动作或状态。 sat down 是一个常用短语动词，字面意思是"使坐下"，在这里指父母让说话者坐下来进行严肃谈话的场景。