The Meaning of As-Sirat Al-Mustaqim, the Straight Path.

As for the meaning of As-Sirat Al-Mustaqim, Imam Abu Ja`far At-Tabari said, "The Ummah agreed that Sirat Al-Mustaqim, is the clear path without branches, according to the language of the Arabs. For instance, Jarir bin `Atiyah Al-Khatafi said in a poem, `The Leader of the faithful is on a path that will remain straight even though the other paths are crooked.'' At-Tabari also stated that, "There are many evidences to this fact.'' At-Tabari then proceeded, "The Arabs use the term, Sirat in reference to every deed and statement whether righteous or wicked. Hence the Arabs would describe the honest person as being straight and the wicked person as being crooked. The straight path mentioned in the Qur'an refers to Islam.
Imam Ahmad recorded in his Musnad that An-Nawwas bin Sam`an said that the Prophet said,

«ضَرَبَ اللهُ مَثَلًا صِرَاطًا مُسْتَقِيمًا، وَعَلَى جَنْبَتَيِ الصِّرَاطِ سُورَانِ فِيهِمَا أَبْوَابٌ مُفَتَّحَةٌ، وَعَلَى الْأَبْوَابِ سُتُورٌ مُرْخَاةٌ، وَعَلَى بَابِ الصِّرَاطِ دَاعٍ يَقُولُ: يَاأَيُّهَا النَّاسُ ادْخُلُوا الصِّرَاطَ جَمِيعًا وَلَا تَعْوَجُّوا، وَدَاعٍ يَدْعُو مِنْ فَوْقِ الصِّرَاطِ، فَإِذَا أَرَادَ الْإِنْسَانُ أَنْ يَفْتَحَ شَيْئًا مِنْ تِلْكَ الْأَبْوَابِ قَالَ:وَيْحَكَ لَا تَفْتَحْهُ فَإِنَّكَ إِنْ فَتَحْتَهُ تَلِجْهُ فَالصِّرَاطُ: الْإِسْلَامُ وَالسُّورَانِ: حُدُودُ اللهِ وَالْأَبْوَابُ الْمُفَتَّحَةُ مَحَارِمُ اللهِ وَذَلِكَ الدَّاعِي عَلَى رَأْسِ الصِّرَاطِ كِتَابُ اللهِ، وَالدَّاعِي مِنْ فَوْقِ الصِّرَاطِ وَاعِظُ اللهِ فِي قَلْبِ كُلِّ مُسْلِمٍ»

(Allah has set an example: a Sirat (straight path) that is surrounded by two walls on both sides, with several open doors within the walls covered with curtains. There is a caller on the gate of the Sirat who heralds, 'O people! Stay on the path and do not deviate from it.' Meanwhile, a caller from above the path is also warning any person who wants to open any of these doors, 'Woe unto you! Do not open it, for if you open it you will pass through.' The straight path is Islam, the two walls are Allah's set limits, while the doors resemble what Allah has prohibited. The caller on the gate of the Sirat is the Book of Allah, while the caller above the Sirat is Allah's admonishment in the heart of every Muslim.)