What don't you get? You don't get what it is, or don't get why people like it?
What it is, is software to synthesize a singing voice, which was developed by Yamaha. Yamaha then licensed the technology to studios, who sampled real singers and sold "voice packs" as add-ons for the core program. This was all marketed to recording studio professionals, and the voices were just sold as voices: "female voice A" and the like. Then, one company decided there was an untapped market of young home users. They created the voice of Hatsune Miku. But unlike previous studios selling voice packs, they created an entire character around the voice and hired a professional anime character designer to create her image. By creating a character / visual representation for the voice they created a whole new market and pushed sales of the Vocaloid engine software through the roof. So, Hatsune Miku started as a piece of software which is an add-on for the Vocaloid program. The expansion gives you new voices to use for the synthesizer, and has her picture on the cover. That's the whole story. Given the success of the first "character voice" this caused a huge shift in the market, and now no studio would consider just creating/selling a "voice pack" without creating a character to go along with it. You have all the typical franchise merchandizing opportunities then: Hatsune Miku figurines, games, cups, hats, etc.
As for why people like it, it's similar to the appeal of Touhou. Vocaloid is at heart, music creation software sold by Yamaha. The whole thing with characters like Hatsune Miku & Friends was created by smaller companies as a way to sell add-on packs for the program. The companies involved make a wide range of media featuring the characters and their voices. But since the software is also about creating your own music videos at home, a huge amount of fan created content sprung up around the franchise.