"Fukkireta" (also know as "Mischievous Function" or "Ochame Kinou") is a song by Gojimaji-P (Lamaze-P/ラマーズP), and sung by UTAU Kasane Teto. Fukkireta has became a popular meme, with different Vocaloids, UTAUs, anime characters and more dancing to the song. Multiple utaites have covered the song, Lon's cover being the most popular. There isn't much of a way to interpret the song, but sometimes people make it different ideas of what the lyrics mean.
One way to think of it is a girl (Teto) describes her love for a boy she likes. Throughout the song, she mentions "the birthday," either meaning she is planning something special for him on his birthday or vice versa. Another way to think of it is Teto is telling the boy she loves how great her passion is for him.
The popularity of both the song and Gojimaji-P are now widespread. This is one of the songs that made both Teto and UTAU famous.