Settlement is the behind-the-scenes operations process through which buys and sells for stocks are fulfilled.
Due to the complexities of this fulfilment process, many orders settle 2 business/working 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 2 business days letter, 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'.