Can someone please tell me what this animes' about?

Actually, Vocaloid isn't an anime! Nor does it have a plot!
It's a computer program that you can make songs with with voicebanks of certain prorgram thingys called "vocaloids". There are already thousands of songs made from the program :3
There are several vocaloids out there(like Miku Hatsune, LOLA, SeeU..) and there are 3 versions: Vocaloid 1, Vocaloid 2, and the most current one, Vocaloid 3. You can buy one from one of these versions and there are Vocaloids for other languages; there are English, Japanese, Korean, Chinese, Spanish, and a French one is coming up ^^ But you can play around with the voices to make them sing different languages, but it may be hard(at least I think, haven't been around in this fandom for a few years)
Hope it helped!

Vocaloid itself isn't an anime. Despite this, the Vocaloids have appeared a few times in the fandom.

This is most recent to...not?

There is a song series sung by Miku and IA known as the Kagerou Project. This was made into a novel, the manga is in progress, and there is an anime that will be released this year.

Miku sung an ending for a 2013 anime, Watamote. It was the ending of the 6th episode.

In the OVA of Lucky Star, Kagami Hiiragi unintentionally wears a Miku outfit.

There are some other instances where Vocaloid is shown or referenced to IN anime, but Vocaloid itself is not an anime.

Below is Kagami dressed as Miku.