Once trades are placed, they settle 2 trading days afterwards. For example, if you placed a trade request on Monday to sell 4 shares of XYZ, the trades would settle on Wednesday (assuming no exchange holidays). 

If you sell a stock the proceeds are added to your "Available to Trade" balance right away and if you buy a stock the stock is available to sell right away. However, if you want to withdraw the funds after you sell a stock, you have to wait for the 2 day settlement period to pass before the funds are added to your "Available to Withdraw" balance.