When used, you will spend all your gold to summon a copy of a random monster from your deck that has a cost equal to the amount of gold you spent. If you have no monster with that cost, it will summon a copy of a random monster from your deck with a lower cost. This doesn't have to be the most expensive card possible, so for example, you will not always summon a Chara if you have one in your deck and use Shopping while having like 50 gold.
Keep in mind that it will takes in account cost changes like Slowing and that you will NOT activate your passive from doing this despite the card saying you "spend" your gold instead of "decrease" it like Mettaton EX's effect says.
This can be helpful alongside cards like Fortune and Burgerpants, who will give you money so you can manage what monster you want to summon. Break can also help to give room on your board as well as give slightly more G.