The circuit is designed to turn the relay on and hold it ON for one second after seeing a positive going input.
To explain that a little better, imagine that the relay is connected so it feeds power to a car headlamp washer.
Suppose that the input is connected to the windscreen washer switch. Each time the windscreen washers are turned on, the circuit receives a 12 volt input.
The timer I.C. will turn the headlamp washer on for just one second then turn off, regardless of whether the windscreen washer is still on or not.
The coupling capacitor, C4, requires a certain amount of time to discharge after seeing an input. So the relay can not be operated again by the headlamp washer switch until a short time has elapsed. This system removes the possibility of having the relay continually clacking on/off as the windscreen washer switch is operated.
The circuit has other uses, too.
The relay can be made to stay ON longer by increasing the value of R2 and/or C2 (which should be a Tantalum type).
Note that not all "555" timer I.C.s will operate satisfactorily in this circuit. The TLC555CP is recommended.
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