As explained in another answer the
:= is a way of forcing a variable to only take on values of a certain type (such as bool, int, float, String, Vector2, Array, etc).
Since that's already been explained I'd like to add that there are two primary reasons for using typed variables.
Reason #1 is that it helps catch coding errors, and makes code more understandable (albeit also longer).
Reason #2 is that it allows the engine to run more efficiently, since the engine knows beforehand what type of data each variable will definitely be.