The diary had slipped between the mattresses and was lost for days.

slipped 是动词 slip 的过去分词形式。slip 的基本意思是"滑落、滑动"，指物体由于失去支撑或摩擦力而意外移动。这里 slipped 表示日记本"滑落"到床垫之间的动作。