What is the knowledge, the learning of which is considered an individual obligation (fard ayn), and does this include deeply understanding (tafaqqul) prayer and fasting, or is knowing how to perform them sufficient?

Individual obligation (fard al-ayn) concerning knowledge is what a Muslim must learn to correct their creed, worship, and profession, such as the six pillars of faith, and how to perform ritual purity (tahara), prayer (salat), and fasting (sawm). To correct one's faith, it suffices to utter the two testimonies of faith (shahadatayn) and to firmly believe in everything brought by the Messenger, peace and blessings be upon him, without needing to learn the proofs. One must inquire about anything that becomes unclear to them.