y is an object's y-location or vertical position. In 2D, the y-location is counted in pixels. Even though y can contain a number with a decimal fraction (i.e. 5.2), it will only take into consideration the whole part of the number.
Example of Use
y contains the vertical position of an object in relation to the current room and never the view. If you wanted to put an object 50 pixels from the top side of a room, you could use the below code.
y = 50;
y = random(room_height);