Paradise Lost is about Adam and Eve's loss of Paradise; their eating of the Forbidden Fruit has often been called the "Fall" (as in, "fall from innocence" or "fall from grace"), so it's no surprise that images of falling occur throughout the poem. The first characters we meet – Satan and his legions – are newly fallen, both morally (they disobeyed God and attempted to overthrow him) and literally (in Book 6 they actually fall out of Heaven and into Hell). Satan's first words to his legions are: "Awake, arise, or be forever fallen." To be fallen, in this poem, is to have sinned, or to have disobeyed God.
It is important to note that in this poem people make themselves fall; there is no "fate" or other force that causes Adam, Eve, Satan, and the rebel angels to fall. Thus, in Book 3 God says that he created Adam "Sufficient to have stood, though free to fall" (3.95-96, our emphasis). And in Book 6, God doesn't push Satan and his legions out of Heaven. They actually throw themselves out of Heaven. Let's repeat that: God doesn't throw them out, they throw themselves out! In Milton's words: "headlong themselves they threw/ Down from the verge of Heav'n" (6.864-865).
In addition to these two falls, Milton also uses a number of other connected images. Now, an object that has fallen is no longer standing; it is no longer upright. In Book 11, Michael tells Adam that "man's woe" (11.632) begins with "man's effeminate slackness" (11.634). Michael implies here that Adam was effeminately "slack" when he listened to Eve and ate the fruit. In other words, he implies that Adam was like a woman, not a man (it is hard not to associate "slackness" with something that is supposed to be hard but is not – wink, wink). Just remember that "slackness" is associated with something going limp, with not being a 'real' man, with the Fall, and with a general lapse in judgment.