SatyaVeda: All about the Veda

 Gayathri Chandah: An instruction telling us which chandas (or metre) to sing the prathama suktha in.

This is generally considered an instruction telling us which chandas (or metre) to sing it in. The word chanda also means pleasing and alluring and can also mean desire or longing. The best way to know what a metre is, is to listen to it. Listen to the Prathama Suktham in Gayatri chandas at this link. It will also help you with the pronounciation.

In present times, Gayathri Devi is worshipped as a goddess who is the essence of all the vedas and who will protect those who sing the Savitr Gayathri Mantram. The word gayathra is translated as song: that which is sung. It is possible that the gayathri chandas was considered a singing chandas.