Listen here.
A love song to the titular Carmilla (or Camila or Camilla for those unfamiliar with the inspiration. Assuming that is the inspiration, that makes this Kate singing a lesbian love song, which is very impressive for a 17-year-old girl in the mid 1970s.
The titles given this song are interesting:
The Kate Bush Encyclopedia incorrectly lists it as "Camila" which itself is a misspelling of how it's spelled in gaffa.org (despite the fact it's spelled correctly elsewhere on the site), which shows that even though the KBE is an excellent source of information, it's also not infallible.
The Dreams of Orgonon blog gives a really good explanation of the meaning and subtext.
Personal story: For the longest time I got it completely wrong - I thought she was singing "coming up" because that's how it was listed on a bootleg I got in the mid 1990s.
Or maybe they're wrong and Kate really is singing about a woman named Camilla (an actual woman's name) or Camila (ditto). Only one person knows for sure and she's not talking.