Galdr: the chant the old poems name
Galdr is a chant, and the word says so itself: Proto-Germanic galdraz, from the verb galana, to sing or to crow. The same root gave Danish and Norwegian gal and Swedish galen, meaning mad, first of all of someone whose mind a song had twisted. The old poems name galdr at work. Odin counts eighteen chants in Hávamál, and Skirnir compels Gerd by chant in Skírnismál. Oddrún sings to help at a birth, Gróa sings nine over her son in Grógaldr, and Beowulf knows Old English gealdru over the dragon hoard. There is even a metre for it: galdralag, kin to the chant metres of the Edda, with an added line and a deliberate parallelism.
Where this comes from
attested use galdr named in Hávamál and the poems of the Edda
The link to runes is attested, and that is rare in this field: chants written in runes exist on staves and plates, the Högstena bronze plaque among them. The plaque is the twelfth century and the younger row, not the twenty-four of this site. What no source carries is galdr sung over the Elder Futhark. The modern habit of singing a rune to wake it is a practice of our time, described here as such. Whether the painted Icelandic staves continue medieval galdr is argued, and this site names it as argued.
What the poems say
The four witnesses never chant: they describe the runes one at a time, and the chants live in the poems of the Edda. What each rune means on its own is gathered here, rune by rune.
Rune magic: what is attested, piece by piece How Odin got the runes