how to make a dragable object with a grid?

The easiest way to do this is math.floor() or math.ciel()

local Fruittilium = game.Workspace.Fruittilium
local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local down = false
Mouse.Button1Down:connect(function()
    down = true
    if Mouse.Target == Fruittilium then
    local Fruit = Fruittilium:Clone()
    Fruit.Parent = game.Workspace
    Fruit.Name = "ACOFruit"
    Fruit.Anchored = true
    Mouse.TargetFilter = Fruit
while true do
    Fruit.Position = Vector3.new(math.floor(Mouse.hit.p.x), math.floor(Mouse.hit.p.y), math.floor(Mouse.hit.p.z)) -- Adding the math.floor() will round it down to the nearest whole number therefore placing it on a stud. Unless you want it to be in the exact center in which case you would need to add or subtract 0.5 from each number.
    if down == false then
    Fruit:Destroy()
    Mouse.TargetFilter = nil
    break
end
    wait(.01)
    end
end
end)
Mouse.Button1Up:connect(function()
    down = false
end)

Hope this helps! If you have any questions feel free to ask.