Like many sura, the Quran here introduces a term (Al-Qaria, meaning "the Clatterer"), and asks itself (and the reader) what it means: "The Clatterer! What is the Clatterer? And what shall teach thee what is the Clatterer?". The answer is said to be a specific day, which has not yet come to pass. On that day "men will be like scattered moths, and the mountains shall be like plucked wool". This day is presumably the Day of Judgment. The image is of chaos and collapse, both of the social order of humanity, and of the cosmological order of the universe itself, as in the Qur'an's usage, the mountains are the pillars separating Heaven and Earth.
The sura continues with this theme of balance and unbalance, by proclaiming that those who have worked to keep the right order of things will be rewarded, and those who have ignored the order of the universe (and presumably those who have intentionally undermined it) shall be cast into "the womb of the Pit". The sura ends as it began, asking about how one will learn the meaning of the word "the Pit" - which is explained to be known in "the blazing fire" of Hell.
It should be understood, however, that translation of this sura, like that of the Qur'an itself, is extremely controversial. Many of the words can be translated in a number of ways, and the original Arabic admits of linguistic ambiguity that is difficult to replicate in other languages. For an interesting alternative translation that preserves some of the original Arabic along with English commentary, see Michael Sells' "The Generous Qur'an."
The Calamity! (1) What is the Calamity? (2) Ah, what will convey unto thee what the Calamity is! (3) A day wherein mankind will be as thickly-scattered moths (4) And the mountains will become as carded wool. (5) Then, as for him whose scales are heavy (with good works), (6) He will live a pleasant life. (7) But as for him whose scales are light, (8) The bereft and Hungry One will be his mother, (9) Ah, what will convey unto thee what she is! - (10) Raging Fire. (11)