Dev Points are needed to unlock items on the island, such as Machines or Trees in the store.

To find out how many Dev Points you have, find this icon under your purple Level Star at the top of the screen, and it will display your Dev Points. Another way to check your Dev Points is to tap on the avatar of your farm - in the new window find the "Island Farm Dev Points" section. It will display your Dev Points and its limit.

You can collect Dev Points on the Island Farm by completing helicopter orders and certain missions on the Island Farm.

First tap on the Helicopter. Choose an Order and produce all the needed items. Once you have all the needed items, they will be colored. Tap in the middle between the two items and hold and drag the items to the helicopter. (Tap the '?' on the top of the panel to see the tutorial).

The helicopter will take all the products and deliver them.

When your helicopter comes back, you'll see a small icon above the helicopter - here are the Dev Points. Tap the icon to collect the points and rewards.

Why don't I get Dev Points from Helicopter Orders anymore?
You start with a maximum limit of 5000 Dev Points on your Island Farm. Once you have reached the limit of 5000, you will get Helicopter Bags instead of Dev Points by completing Helicopter Orders.
The limit can be increased by building Landmark Decorations from Decorations -> Landmarks in the Island Farm Store. Each decoration can be purchased multiple times but it will only increase the maximum Dev Point limit the first time you build it.
Please note: Landmark Decorations do not give any Dev Points, but only increase the maximum Dev Point limit. After increasing the limit, you can continue to gain Dev Points from Helicopter Orders.