Mom helped me tuck the edges of the blanket under the mattress so I wouldn't get cold at night.

tuck 是动词，意思是"把...塞进、掖好"，指将某物的边缘或末端整齐地塞入或折叠到另一个物体下面，使其固定不松散。在这里指把毯子的边缘塞到床垫下面固定住。