What is this symbol used in this D flip flop circuit?
Thats the symbol for a transmission gate.

It's basically a switch controlled by a digital input. It consists of a CMOS-pair for the actual switch and another pair for an inverter for the control signal. The inverter is needed because the P-MOS and N-MOS transistor need inverted levels to turn them on. 

Note: the whole latch consist of two transmission gates and two inverters.

A transmission gate can be used as an analog switch but of course it also works with digital signals like in this case.

Note also, to be exact: the circuit shown is not a D-flip-flop as stated, but a (transparent) latch.