"Very" ... according to most of the definitions I've read for the word "song," if a musical piece doesn't have any words included in it, it's called music. The words don't have to be human though, it seems, as the sounds made by animals qualify as turning music into a song ... like the song of a bird or whale.
So when someone asks you how many songs you have on your computer, you shouldn't include the instrumentals.
To me, just as important as the melody and arrangement. If I can't hear the lyrics clearly, I turn off and don't listen at all. The lyrics I like most are poetry in their own right. They cover a wise range of topics, use language with exquisite beauty, and express truths of human experience. The ones I detest glorify dysfunctional relationships.