Trade date (T) represents the day your order to buy or sell a security is executed; settlement date is the day your order is finalized and on which funds and the securities must be delivered. Currently, settlement date occurs two business days after trade date (i.e. T+2). If you sell shares on a stock on Tuesday, the transaction will settle two days later on Thursday.
Under the new T+1 settlement cycle, most securities transactions will settle on the next business day following their transaction date. Using the example from above, if you sell shares of a stock on Tuesday, the transaction will now settle on Wednesday.