In-stock cars are pre-built and either held by a dealership in their showroom or are due to arrive with them shortly. They are generally available sooner than factory order cars and can sometimes come with higher discounts too.
There are various types of stock car, including:
New - brand new and unregistered, just like a factory order.
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 log book.
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.