Settlement is the behind-the-scenes operations process in which buy and sell orders for stocks are fulfilled.
Due to the complexities of this fulfilment process, many orders settle 2 days after they were made (AKA T+2) as industry standard.
To an end customer, settlement doesn't have that much of an impact. Once your order has gone through with a broker, that's a legal contract and the order will be fulfilled at the price you saw.
That's why if you execute a sell order, the proceeds will go into your available cash as 'unsettled cash'. This can be re-invested on Freetrade as soon as you like.
However, until the cash settles, you can't withdraw the cash into your bank account. When it settles, the money will leave 'unsettled cash' and go to into your 'withdrawable cash'.