The Dream of the Rood, like many Anglo-Saxon poems, is unsigned, and exists in a single copy, found in the Vercelli Book. The Vercelli Book is one of only four known collections of Old English poetry, most of whose entries are anonymous. It was discovered in 1822 in the city of Vercelli Library in Italy.
In the poem, the poet dreams in vivid imagery of a tree and the rood/cross. ("Rood" is from the Old English rōd meaning "pole" and is usually used to mean the crucifix.) He imagines a bejeweled cross that transforms into the plain wooden crucifix representing Christ's suffering. The cutting down and cutting up of the tree to make a rood is compared to the suffering of Christ. The rood itself speaks in the dream, telling how it received the body of Christ, saw his suffering, then was cut down and buried, only to be dug up and adorned with gems.
This story seems to be tied to the idea that Helena dug up the True Cross. Coincidentally, the Vercelli Book includes a poem, Elene, which tells the same story.
The date of them poem isn't certain, but there is a carved stone cross, the Ruthwell Cross, on which are inscribed lines reminiscent of several lines from the poem. The Ruthwell Cross was erected in the 8th century. If the lines were not carved at a later date than the creation of the Cross, then we know the poem was around prior to that, and known to people.
We should probably take a closer look at the Ruthwell Cross next. In the meantime, if you wish to read the poem (in Modern English, that is), here's a link.