How do I make a sliding door?

As thumb rule you can say that you can make everything. Some things are technically impossible and some things are possible but will overwhelm CPU resources.

That's absolutely not the case in this. Basically, you have rocks, which you could say are several doors which you are sliding to the right. This is very objective though as I don't really know what to "right" is, it depends on your viewpoint.

I would create it as this:

function drag(doors)
    for i = 1,30 do
        for i, rock in pairs(doors) do
            rock.CFrame = rock.CFrame * CFrame.new(0,0,0.2)
        end
        wait()
    end
    wait(5)
    for i = 1,30 do
        for i,rock in pairs(doors) do
            rock.CFrame = rock.CFrame * CFrame.new(0,0,-0.2)
        end
    end
end

local doors = {}
table.insert(doors, game.Workspace.Rock1)
table.insert(doors, game.Workspace.Rock2)
-- and so on
-- an event happens:
drag(doors)

You will notice however that the doors are sliding to the left, or in the wall, or from it. You need to either change the CFrames to one of the following:

CFrame.new(0,0,-0.2) and CFrame.new(0,0,0.2)

or

CFrame.new(0.2,0,0) and CFrame.new(-0.2,0,0)

or

CFrame.new(-0.2, 0, 0) and CFrame.new(0.2,0,0)