When the clock input on the D-type flip-flop changes from '0' to '1', the data on the 'D' input is transferred to the 'Q' and '~Q' outputs.
The device acts as a latch, so that the 'D' input may change but the output remains constant until the next clock edge.
You can use Set ('S') and Reset ('R') control pins to put the output in a predefined state.