In-stock cars are cars that are pre-built and either held by a dealership in their showroom or are due to arrive shortly. These cars are generally available much sooner than factory ordered cars and can sometimes come with better savings as well.
There are various types of stock cars, including:
New - brand new and unregistered, just like a factory ordered car.
Pre-reg - previously registered to the dealership but unused with delivery mileage only. Large savings can often be had on these cars. Just remember you will be the second owner on the logbook.
Ex-demo - previously registered to the dealership and used by them as a demonstrator for their customers. You will be the second owner and the car will have some mileage.
Used - previously owned and driven by somebody else. These used cars will undergo the dealer's pre-sale checks prior to purchase.
You can view cars currently in-stock here, once logged in to your account.