First: he meant that it should be light when the prayer is completed. In other words, prolong the recitation until you finish the prayer when it has grown light. The Prophet, may Allah bless him and grant him peace, used to recite between sixty and a hundred verses in it—roughly half a hizb.

Second: he meant that dawn should become clear and distinct, so one should not pray merely on the basis of a strong supposition that it has begun. The Prophet, may Allah bless him and grant him peace, used to pray after dawn had become distinct, except on the day at Muzdalifah, when he offered it earlier than was his custom. And Allah knows best.

He—may Allah have mercy on him—was asked:

A man from among those who profess Islam abandoned prayer for two years, then repented and began performing it regularly. Must he make up the prayers he missed or not?

He replied:

Someone who omits a prayer or one of its obligatory elements may have done so out of forgetfulness, having previously known it to be obligatory; out of ignorance of its obligation; because of an excuse which he believes permits delaying it; or knowingly and deliberately.

As for someone who forgets a prayer, he must perform it when he remembers it, in accordance with the Sunnah of the Messenger of Allah