A vector3 contains an x, y, and z value. By giving values for x, y, and z, you can use this to determine the "direction" of the vector.
Vector3(1, 0, 0) is a vector pointing at the positive x direction
Vector3(0, 1, 0) is a vector pointing upwards in the y direction
Vector3(1, 1, 0) is a vector pointing both in the forward x and positive y direction. (And as x and y are equal this will be at a 45 degree angle)
The only issue with the last one is that - because of Pythagoras' theorem - it has a length of sqrt(2), not 1. By "normalizing" the vector, this essentially turns this vector into a length of 1, so the speed will remain constant for any direction.
Syntax in godot is Vector3(x, y, z).normalized()
Hope that clarified a little