Holding stock is the more traditional way to do business. You buy the products, have them delivered to you, and keep them in your warehouse.
When you get an order, you ship the product directly to the customer.
Dropshipping is the opposite. You have a relationship with a distributor and add the items to your website.
Only when you get an order, do you pass that order onto the distributor and they ship the products directly to your customer.
As I mentioned, there are pros and cons to both methods.