Setup, Hold time & metastability of a flop.

Setup time - Setup time is measured at the input of the flip-flop with respect to rising/falling edge of the clock to the flop. The time signifies the minimum duration of data stability before the arrival of rising/falling clock edge. With this requirement the flops will reliably sample the data at the output.

Hold time - Hold time is measured at the output of the flip-flop with respect to rising/falling edge of the clock to the flop. The time signifies the minimum duration of data stability at the output after the rising/falling clock edge. With this requirement the output flip-flop data is stable enough to drive the digital logic.

Metastability - It’s a condition on the output signal of a flip-flop due setup or hold time violations discussed before. 

A metastable signal does not represent a high ’1’ or a low ’0’ and results in unstable output or a glitch to the digital circuit.