As the docs say here That method retunrs a vector with length 1 that points to the Vector in the argument. For example:
var start = Vector2(3,4)
var end = Vector2(5,5)
var direction = start.direction_to(end)
# will print (0.894427, 0.447214), as it is the normalized vector
# that points to `end` vector, starting from `start` vector.