Some of the runes are lines from the Anglo-Saxon poem The Dream of the Rood, giving us an opportunity to date the poem. The cross dates from the 8th century, which lets us know that the poem must have been well-known enough by then to be considered appropriate for carving.
Sadly, it was smashed in 1642 by Presbyterian iconoclasts. Fortunately, after smashing it, the destroyers left the pieces lying there, enabling a Scottish minister to re-assemble it 1818. Weathering and the destruction has obscured the carvings a little.
Besides traditional vine-scroll designs of leaves and birds, scenes show Mary Magdalene washing Christ's feet, Christ having dominion over the animals, Saints Paul and Anthony breaking bread in the desert (that one has a carved inscription that makes it clear), the healing of a blind man from the Gospels, and more.
The Ruthwell Cross is not completely unique, in the sense that there is another "cross" of similar vintage and style. We will talk about that next.