This is a vector variable. You can declare variables under the Variables tab on the left hand side of your Blueprints:
A shortcut you can use is to press and hold CTRL on your keyboard and left-click drag the variable onto the graph to create a getter, or press and hold ALT on your keyboard and left-click drag the variable onto the graph to create a setter.
(I have a special plugin that changes the look of the nodes, so don’t worry if they look slightly different).
When you have any type of variable, you can use a getter or a setter: a getter gets the current value of the variable and the setter variable sets it to a new value.
You can find more information in this documentation under the title Accessing Variables in Blueprints: Blueprint Variables | Unreal Engine Documentation
I hope this helps, and good luck 
