Hi Sindhuri
You can google it ! you'll find gang of pages about this issue . but i prefer to tell you a bit about it !
A triac is an alternative switch which can be turned on when command signal is incoming and it will be on till the voltage across MT1 and MT2 be lower than a specific value ( or perhaps current ) .
You can use from this property and then by changing the situation of command signal , you'll change the average of your signal or as a matter of fact if you're giving it to a bulb , you'll be able to control it's light strength !
Best Wishes
Goldsmith