How is this done?

um.. why move every tile? use the SetOrigin(x,y) command. That way you just load your tiles at their normal x/y coordinates. Then you can simply have a character that moves only their cooridnates.

psuedo code is something like

global player:MyPlayer = new MyPlayer
loadTiles()

while (running)
	cls
	update()
	flip
wend


function loadTiles()
	'load tiles how ever you want to load them.
	' just set their corresponding x/y coordinates
	' tip: it'd make it easier if you made a Tile an actual custom Type with x/y fields.
end function

function update()
	xMove = 0
	yMove = 0
	
	' just listen for up and right arrow key hits
	if keyhit(KEY_UP) then
		yMove = - 5
	end if
	
	if keyhit(KEY_RIGHT) then
		xMove = 5
	end if
	
	' move the player in that direction	
	player.move(xmove, ymove)

	' now move the drawing origin in the opposite of that direction
	' so you can still see the player, but the entire world/tiles
	' will shift.
	xorigin:int = 0
	yorigin:int = 0
	getOrigin(xorigin, yorigin)
	xorigin:- xMove
	yorigin:- yMove
	setOrigin(xorigin,yorigin)
end function

type MyPlayer
	
	field xPosition
	field yPosition

	method move(x,y)
		xPosition:+ x
		yPosition:+ y
	end method
end type

the advantage? you don't have a nasty for loop moving every object in your game world by some small amount of pixels. With this method you have something of a very basic 2D 'camera' of sorts that you cam move around with setOrigin().