Cerdd Dant has a very long history in the Welsh tradition. In essence it is the art of presenting or performing poetry to harp accompaniment. In the old days, the poems sung were in cynghanedd, the strict Welsh metres based on alliteration and complicated rules, but later on the free metres were used just as much.
How does it work? The harp always plays a set melody – a traditional melody or a melody composed in the traditional style. The singer waits for a few bars and then sings his or her words on a counter melody, ensuring that the main accents of the metre fall on the main accents of the harp melody. The singer and harpist both have to end each verse together: the last word of each verse always falls on the main beat of the last bar in the harp melody.
The counter melody in the old days was always improvised, but is almost invariably set in advance and learnt by modern singers. This counter melody is called a cyfalaw. The art of composing the cyfalaw and placing all the words in the right place is called gosod. The harp melody is called the alaw or the cainc.
Source: https://www.cerdd-dant.org/about.html (Cerdd Dant Society)
Here are some fine examples of cerdd dant.
https://www.youtube.com/watch?v=q_OsHvUSp6U – solo by comedian Ryan Davies
https://www.youtube.com/watch?v=GqubDdbn868 – Côr Cerdd Dant Agored
https://www.youtube.com/watch?v=Ahy-pHzMvOQ – Côr Seiriol