This line assign the x property of input_vector to the “right” input minus the “left” input. get_action_strength() returns the “axis strength” of an input. If “right” is a key input, its strength will be 1 (when pressed) or 0 when not. If it were an analog joystick, on the other hand, the value will be something between 0 and 1, depending on how far the joystick is moved.
Subtracting left from right means the value of this is going to end up being 1 (if just “right” is held), -1 (if just “left” is held), or 0 if both are held.
(NOTE: Missing () added) This line takes the resulting value and normalizes it. Normalizing a vector means setting its length to 1 while keeping the direction the same. This is used so that diagonal movement (“down” and “right”, for example), won’t be faster, since the length of (1, 1) would be 1.414.