We could say "That wine is an excellent wine." talking about a specific wine. Otherwise we could say "That wine is excellent."
When we say "It is excellent wine.", we are talking about an undetermined amount of wine perhaps speaking of a particular year of a particular wine, but not about a particular bottle. While "It is an excellent wine." is likely about one bottle of wine.
um = a, an, one
one = um, uma
a, an = um, uma
Portuguese here is different from English, since the Portuguese article 'um' also is the number 1. In English, 'a' and 'an' are articles that mean 'one', but the number 'one' is a different word, not the same word as the article.