My son frequently ask questions about the couple kissing on the TV show last night , Like dad why they are kissing? etc . How would I make him understand in the best way ?

First of all, how old is your child? Personally, I think there's nothing wrong if you are going to educate your kid about that kind of stuff at his early age. Make him understand that kissing is a way of expressing love to your partner. You can set you and your wife as the perfect example. Initiate a discussion where he would get the sweet purpose of the gesture. :)