What do the Houthi attacks on Saudi Arabia mean?

How the Houthis are able to carry these attacks is a mystery to To Vima:

“Most analysts say that the pro-Iranian rebels could not have built up a stockpile of precision-guided weapons with a range of 1,500 to 2,000 kilometres without Tehran’s assistance, so without the support of Iranian technical and military advisers. However, they struggle to explain how this vast arsenal, which had been hidden all along, was created, or how the Houthis are able to breach the missile defences of the Saudis and their allies in so many places and with such frequency.”