I know that disobedience to parents is the second major sin - but may I know what exactly does disobedience to parents include? Does it also include going out without parents' permission to Islamic schools for the purpose or learning or teaching the Qur'an and Islamic knowledge? Does it also include marrying any person whom you don't like but the parents like and they want you to get married to him just because they like him? If we refuse to marry a person of their choice, then they say that you are being disobedient to your parents. Is that true?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Obeying one's parents is one of the best ways to get closer to Allah and disobeying them is one of the major sins.

But making efforts to learn some obligatory knowledge without the consent of one's parents is not a form of disobedience.
If the knowledge the person wants to acquire is not obligatory, then he should try to convince them in a kind and gentle manner.

As for refusing to marry a given person, it is not a form of disobedience and it is unlawful for parents to force their children to marry someone they do not like.

For more benefit on the limits of obedience to parents, please refer to Fataawa 271255, 131695, and 123554.

Also, for more benefit that parents don’t have the right to force their children to marry someone they don’t like, please refer to Fataawa 87750, 92731, 86777, and 134358.

Allah knows best.