Ave, Regína cælórum,
Ave, Dómina Angelórum:
Salve radix, salve porta,
Ex qua mundo lux est orta:
Gaude, Virgo gloriósa,
Super omnes speciósa,
Vale, o valde decóra,
Et pro nobis Christum exóra.
I am having trouble deciphering the bolded line of this hymn, especially the word decora. Is it an noun, adjective, or verb? The o makes me think it is either a noun or an adjective acting as a substantative, but then what is the adverb valde doing here?