Basically, the above answer is a good summary. I will add a few observations to it. The poem is a type of fable, a tale that uses animals to develop a moral lesson. You can tell by the examples of alliteration, such as "Bingle Bog" and the rhyming nature of the poem that it is intended to move swiftly and to draw in children.
However, the moral is one for all ages. The frog is the evil villain. He is rude, manipulative, greedy and without emotion. The nightingale is the innocent and naive victim. Villains hate beauty, and the frog is no different. He hates the beauty of her singing and is jealous of the attention her singing brings to the bog. He decides to capitalize on the situation by pretending to befriend the nightingale, who has never been subjected to this kind of manipulation. He charges admission and continually scolds the poor bird, pushing her until she dies.
Notice the animals have no names. The nightingale can represent anyone with a true talent and love for something. The frog is the person who uses this talent for his own means. Think of a parent whose child is gifted in a sport. How many times can that parent scold and push that child to hate a sport he once loved?
This is the moral of the poem
The poem frog and the nightingale is written by vikram seth (as you know it). This tells us the story of two characters; that is the frog and and the nightingale. once there was a frog which lived in bingle bog. Day in and day out the frog kept singing on and on and on..... .Other creatures around the bingle bog hated his voice .Even if they throw sticks or bricks at him he would never stop singing this potrays that the frog was very overconfident about his voice. But one night a nightingale sat on the sumac tree and started to sing,all the creatures around the bingle bog rushed towards the nightingale's melody and stared towrds the nightingale with great attention. Once the nightingale had finished her song every one had appreciated it, except the frog, the frog did not like how every one caught it's attention this made the frog jealous.So the next mornng when the nightingale was about to sing the frog interupts her and decides to introduce himself as the great king of the bog.The nightingale asks the frog how was her song? the frog doesn't acknowledge her. he then offers to train her, the foolish nightingale accepts his offer. The next day the nightingale sang to the audience and she became a huge sensation. But the clever frog took advantage of this situation and made money out of it. The frog then forced to sing her in horrid weather thus she lost the quality and charm in her voice. the frog finally wnted to get rid of her so he scolds even further asking her to sing the brainless bird afraid to fail puffs her lung bursts a vein and dies.In the end the frog remains insensitive.