All instances in GameMaker: Studio have certain "built
in" properties that you can use and set to govern how they look and
behave. hspeed is one of those properties and defines the
horizontal movement speed of the instance in pixels per step. So,
an hspeed of 3 means 3 pixels of movement to the right
(+x) every step, and an hspeed of -3 would mean 3 pixels
of movement to the left (-x) every step.
if keyboard_check(vk_left) hspeed = -5;
if keyboard_check(vk_right) hspeed = 5;
The above code will change the horizontal speed depending on which keys are pressed.