What does 再不起床太阳就要晒屁股啦 say?
再不起床太阳就要晒屁股啦。
  If you don't wake up, the sun will shine on your bottom.


It directly breaks down:


再不 = "if not"
起床 = "wake up"
太阳 = "the Sun"
就要 = "be about to"
晒 = "to shine one" (e.g. used for sunbaking)
屁股 = "bottom" (i.e., the body part)
啦 has no real meaning; it makes it sound cute


So that's its literal meaning, but why is this important?


  In ancient times, people paid attention to sleeping [while] facing south.  So when the Sun rises from the east, the Sun won't shine on your bottom.  When we reach noon, in the environmental timetable of the Central Plains, the Sun will only then shine on your bottom.
  古代人讲究睡觉坐北朝南。那么日出东方时太阳是晒不到屁股的，只有正午时，在中原地带的时间表环境下，太阳才可能晒到屁股啦。
  iTalki


So the sun shining on your bottom literally implies it's noon.  However, it's metaphorically used to refer to someone who is sleeping in; perhaps they're being a bit lazy, and perhaps they're a bit cute.