She's a vocaloid, a voice synthesizer that sings that is also depicted as an anime character. There are many kinds of them and if you hang around some part of the internet long enough, you will learn some of the songs. I'm not really a fan of her, but on top of my head some popular song she sang are Miku Polka( I never learned the actual song name), the Popipo commercial song and her collaboration with Domino's Pizza, of all things.
Despite her original intent, she is depicted in many commercials, even anime and video games!
She is quite popular and personally speaking I only knew her from the random memes that pop up every now and then. Oh yeah didn't she get into Fortnite recently?
Hatsune Miku is a voicebank for the voice synthesis software "VOCALOID". It's very complicated to explain but basically it's a software where you can place down different notes with certain hiragana and it will make the character sing !
Miku, being a voicebank for the program, had a huge surge of popularity in the 2010s and is kind of an iconic symbol for online anime spaces.
She also has her own games!! The most prominent being the ones in the "Project Diva" series and the mobile game that released in 2020 "Project Sekai".
