睡熟 means 'to sleep soundly' or 'to be in a deep sleep.' This is a verb-complement structure (动结式) where 睡 is the action and 熟 indicates the depth of sleep. It can be split to insert 得 (indicating possibility/degree) or 不 (impossibility): 睡得熟 ('can sleep soundly') and 睡不熟 ('can't fall into deep sleep'). The character 熟 is often pronounced shóu in colloquial speech (as reflected in the pinyin here), though in formal contexts shú is also common. This is a separable pattern, so be comfortable placing other words between the two characters.