Is it obligatory for a woman to cover what is beneath her chin during prayer, and what is the ruling on the prayer of one who does not conceal this area?

The term "face" refers to what enables confrontation (mutual facing). Jurists have differed in its definition. Al-Shirazi and Al-Nawawi stated that the face in wudu' (ablution) and salat (prayer) is the area from the hairline to the chin, and from ear to ear. Al-Kassani, on the other hand, mentioned that the face is the name for what a person faces or is faced with.

Based on this, the area below the chin is not part of the face and should not be uncovered in prayer, because it is not that which enables confrontation.

If a woman was unaware of the ruling regarding covering the awrah (private parts) in prayer, she is not required to repeat her past prayers. She should repent and strive in righteous deeds.

As for a small part of the awrah being uncovered during prayer, the majority of scholars, such as Imam Abu Hanifa and Ahmad, hold that the prayer is not invalidated by this, and this was also the view preferred by Sheikh al-Islam Ibn Taymiyyah. However, if the uncovering is extensive, the prayer must be repeated within its time according to the four Imams and others. It is better to be diligent in covering all that can be covered, as a way of avoiding differing opinions.