Masum: Sinless, free from sin.
The name of Ahl al Bayt are not limited to Imam `Ali, Bibi Faatima, Imam al Hasan and Imam al Husayn `Alayhimu Ssalaam because Rasul Allah (Salla~llahu `Alayhi Wa Aalihi wa Sallam) is reported to have prophesied the names of the Imams from his Ahl al-Bayt that would follow. In a Hadith reported, he publicly announced them by name; and then each one of them is reported to have announced his successor. The Twelve names that Rasul Allah Salla~llahu `alayhi wa Aalihi wa Sallam announced are: (1) Imam `Ali ibn abi Talib (2) Imam al-Hasan ibn `Ali (3) Imam al-Husayn ibn `Ali (4) Imam `Ali ibn al-Husayn (Zayn al Aabidin) (5) Imam Muhammad ibn `Ali (al-Baqir) (6) Imam Ja`fer ibn Muhammad (al-Saadiq) (7) Imam Musa ibn Ja`fer (al-Kaazim) (8) Imam `Ali ibn Musa (al-Rida) (9) Imam Muhammad ibn `Ali (al-Jawaad) (10)Imam `Ali ibn Muhammad (al-Haadi) (11)Imam Hasan ibn `Ali (Hasan al-Askari) (12) Imam Muhammad ibn Hasan (al-Mahdi) May Allah's Peace be upon all of them as we await the return of our Twelfth Imam, also as per prophesy from Rasul Allah Salla~llahu `Alyhi wa Aalihi wa sallam. By virtue of the above Hadith, they too are part of the Ahl al Bayt. Since they are the Imams appointed, they too are beyond making any sins,and therefore, Ma`sum. As to your question of why are they sinless or what does the sinlessness mean, I think it is very important to know that their sinlessness does not mean that they are not humanbeings like ourselves. We are not allowed to exaggerate anything about Imams and it is our fundamental belief that they are human beings like ourselves and are bound by all the Laws and Hukm of Allah Subhanahu wa ta`ala. They do not commit sins because they are honoured servants of Allah; and Allah Subhanahu wa ta`ala has honoured them and has given them great dignity and authority. Allah has protected them from all kinds of uncleanliness and has throroughly purified them (see Qur'an 33:33). They have the highest perfection and are endowed with `ilm al-ladunni and chastity. They are endowed with all virtues, bravery, chastity and quality. Further, they are the carriers and transmitters of the correct Sunnah of Rasul~Allah. They are therefore our Imams. They are the guides and they have the authority after Rasul Allah(Salla~llahu `Alayhi wa Aalihi wa Sallam) in the matters in which we require help and guidance; in religious commandments, in Judgment, in legislation, in the commentary of the Qur'an and in the interpretation of the Qur'an.