What are Datastores and how do I use them?

A data store is essentially a dictionary, like a Lua table. Each value in the data store is indexed by a unique key, for instance the player's unique Player. UserId or simply a named string for a game promo. To create a new entry, call SetAsync() with the key name and a value

Like this:

-- // Assigning variables //
local DataStoreService = game:GetService("DataStoreService")
local dataStore = DataStoreService:GetDataStore("MyDataStore") -- This can be changed to whatever you want

local function saveData(player) -- The functions that saves data

    local tableToSave = {
        player.leaderstats.Money.Value; -- First value from the table
        player.leaderstats.Coins.Value -- Second value from the table
    }

    local success, err = pcall(function()
        dataStore:SetAsync(player.UserId, tableToSave) -- Save the data with the player UserId, and the table we wanna save
    end)

    if success then -- If the data has been saved
        print("Data has been saved!")
    else -- Else if the save failed
        print("Data hasn't been saved!")
        warn(err)       
    end
end

game.Players.PlayerAdded:Connect(function(player) -- When a player joins the game

-- // Assigning player stats //
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local Money = Instance.new("IntValue")
    Money.Name = "Money"
    Money.Parent = leaderstats

    local Coins = Instance.new("IntValue")
    Coins.Name = "Coins"
    Coins.Parent = leaderstats

    local data -- We will define the data here so we can use it later, this data is the table we saved
    local success, err = pcall(function()

        data = dataStore:GetAsync(player.UserId) -- Get the data from the datastore

    end)

    if success then -- If there were no errors and player loaded the data

        Money.Value = data[1] -- Set the money to the first value of the table (data)
        Coins.Value = data[2] -- Set the coins to the second value of the table (data)

    else -- The player didn't load in the data, and probably is a new player
        print("The player has no data!") -- The default will be set to 0
    end

end)

game.Players.PlayerRemoving:Connect(function(player) -- When a player leaves
    local success, err  = pcall(function()
        saveData(player) -- Save the data
    end)

    if success then
        print("Data has been saved")
    else
        print("Data has not been saved!")
    end
end)

game:BindToClose(function() -- When the server shuts down
    for _, player in pairs(game.Players:GetPlayers()) do -- Loop through all the players
        local success, err  = pcall(function()
             saveData(player) -- Save the data
        end)

        if success then
            print("Data has been saved")
        else
            print("Data has not been saved!")
        end
    end
end)