滚雪球 literally means 'to roll a snowball', but it's almost always used figuratively to describe something that grows rapidly and uncontrollably as it accumulates, like a snowball rolling downhill. It's a separable verb-object phrase (离合词): you can insert a measure word or modifier between 滚 and 雪球, e.g. 滚了一个雪球 ('rolled a snowball'). The figurative sense doesn't split — it stays as a fixed idiom.