Soul sleep is the belief that the dead are unconscious until their resurrection. For advocates of this view, the soul does not exist somewhere in a conscious state during that interval.
Armstrongites like to appeal to Ecclesiastes 9:5 to support soul sleep. They usually quote only part of the verse, the part that says "the dead know not any thing[.]" I'd like to post the verse in its context, then I'll make some comments. I'll use the King James Version because that's the one Armstongites tend to gravitate towards
4 For to him that is joined to all the living there is hope: for a living dog is better than a dead lion.
5 For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.
6 Also their love, and their hatred, and their envy, is now perished; neither have they any more a portion for ever in any thing that is done under the sun.
7 Go thy way, eat thy bread with joy, and drink thy wine with a merry heart; for God now accepteth thy works.
8 Let thy garments be always white; and let thy head lack no ointment.
9 Live joyfully with the wife whom thou lovest all the days of the life of thy vanity, which he hath given thee under the sun, all the days of thy vanity: for that is thy portion in this life, and in thy labour which thou takest under the sun.
10 Whatsoever thy hand findeth to do, do it with thy might; for there is no work, nor device, nor knowledge, nor wisdom, in the grave, whither thou goest.
This Ecclesiastes passage seems not only to dismiss the immortality of the soul, but the resurrection as well. V 4 says that the living have hope, which implies that the dead do not. V 5 states that the dead have no more reward, which contradicts the idea that there's a resurrection in which the righteous will be rewarded. V 6 affirms that the dead have no more portion in anything done under the sun. Would the author of Ecclesiastes say this, if he believed the dead would rise again and once more be able to have a portion of life? (I guess one way to get around this, however, is to point out that the sun will be abolished after the second resurrection; Revelation 21:23; 22:5). VV 7-10 essentially say, "Enjoy life while you can, because it's not going to last forever!" Why would Qoheleth say this, if he thought there would be an afterlife?
Jews have had different ideas on what to do with Ecclesiastes. There was dispute about its canonicity at one point. The Jewish Encyclopedia article on death mentions the following interpretation: "The dead are supposed to take an active interest in worldly affairs. The assertion of Kohelet that 'The dead know not anything' (Eccl. ix, 5) is interpreted. 'The wicked who are considered dead while yet alive'" (Shab. 13b?). Here, Ecclesiastes is treated as inspired, since Jewish exegetes try to reconcile it with their belief in the consciousness of dead people. Essentially, they say that Ecclesiastes 9:5 means that spiritually dead people are pretty dumb.
Sounds interesting, but it doesn't exactly fit the context. Is there a way for Christians to find religious value in Ecclesiastes, without embracing what it says about death being the end? Perhaps. I Corinthians 15 says Christ's resurrection is the guarantee of our resurrection, and Hebrews 12:14-15 states that Christ delivered those who were in bondage on account of their fear of death. Maybe Ecclesiastes can remind us that many people prior to the coming of Christ were afraid to die, and we can then appreciate all the more that Christ destroyed death through his resurrection.