1254/1 - Abu Ayyūb (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: “Whoever fasts Ramadan and then follows it up with six days in Shawwāl, it is as if he has fasted the entire year.” [Narrated by Muslim]
1) A good deed is rewarded with ten times as much. So, the fasting of Ramadan is equal to ten months. The six days in Shawwāl equal sixty days, namely two months. This adds up to a whole year.
2) It states the promised reward for those who fast the entire Ramadan along with six days in Shawwāl even if the six days were not in succession.
Some scholars said that a person may not fast the six days in Shawwāl before he has made up for any fast he missed during Ramadan for a valid reason. This is because the reward in the Hadīth is promised to he who fasts all of Ramadan and then fasts six days in Shawwāl.
Other scholars said, however, that it is permissible to fast the six days in Shawwāl before making up for any missed fast in Ramadan, based on the following statement by ‘Ā’ishah (may Allah be pleased with her): “I used to have missed days of fast in Ramadan, and I would not be able to make up for them except in Sha‘bān.” Yahya, a sub-narrator, said: “Her preoccupation was with the Prophet (may Allah’s peace and blessings be upon him).” [Narrated by Al-Bukhāri]
They said: ‘Ā’ishah (may Allah be pleased with her) did so in the presence of the Prophet (may Allah’s peace and blessings be upon him), and it is unlikely that the Mother of the Believers - the knowledgeable and pious person - would fail to fast the six days in Shawwāl.
Moreover, it is part of the Shariah’s ease and facilitation that it allows us to delay making up for the missed fast of Ramadan, and Allah knows best.