Theory:

A main or central idea is the life of any passage. It conveys the central theme, making the readers feel connected to the author's point of view. It is mostly conveyed in the initial and concluding lines of any paragraph or passage. The main idea is a vital element in any passage or a text.
Dad and the cat and the tree.svg
Example:
This poem 'Dad and the cat and the tree' is written by 'Kit Wright', an eminent British writer. There are 14 stanzas in the poem, arranged in a quatrain format. The poem funnily narrates a day's incident in which a kid will happen to notice an innocent cat stuck on a tree one morning in the garden. This problem motivates the kid's father to take up the initiative to rescue the cat from the tree, despite his wife's warnings. The Child's father is the main protagonist of this poem. He did not lose his confidence even after falling twice while trying to climb up the tree. Firstly, he uses a ladder and then the tree's branch to climb up the tree. His efforts in saving an abandoned cat from the tree fulfil the poem's plot. The poem ends humorously with a funny Climax, as the cat will get down safely to the ground, while the kid's father remains stuck in its place in his third attempt.
Main idea of this passage:
A cat had got stuck in a tree, and the kid's father tries to save it. At last, the cat would have been saved, and the kid's father would have got trapped in the cat's place.
Things to remember:
Kit Wright
British writer
14 stanzas
quatrain format
day's incident
cat stuck on a tree
problem
initiative
rescue
wife's warnings
first, second and third attempt
ladder, tree branch
funny climax
cat saved
dad got stuck
Minor ideas:
1. The kid's mother warned her husband several times.
2. The kid's father had not listened to his wife.
3.  The child's father plays the lead role in this poem.
4. The whole story is woven around the child's father and his actions.
5. The child's father fell twice in his attempts.
6. His first attempt was made using a ladder.
7. His second attempt was made using a tree's branch.
8. His third attempt though successful, remained useless.
9. His third attempt, in spite, landed in trouble.
10. The cat got saved by itself.
11. The Child's father got stuck in the cat's place, unfortunately.
S.No
Words
Meaning
1.
InitialRefers to something that happens in the beginning
2.
Concluding Refers to the end or finishing section
3.
PassageRefers to a paragraph