Retriggerable one shot/monostable multivibrator circuit is a flip-flop circuit which give active output when an input is present, and hold the state for a predetermined period before turns to inactive state. The difference between retriggerable and non-retriggerable  is that retriggerable circuit gives period extension when the input is retriggered before the output changes to inactive state.  Here the circuit uses 556 integrated circuit, which contain two 555 timers in single package IC. Here is the schematic diagram and the input-output timing chart of the circuit:

Retriggerable One Shot circuit schematic

[Circuit’s schematic diagram source:]