Waifu (not to be confused with the male term, husbando) is the word used when somebody is attracted to a female fictional character, such as an anime or a video game character. Waifus are mentioned a lot on Miiverse, with a lot of artists including their waifus in drawings.
Most Miiverse waifus are female Nintendo characters. Basically all female characters have a fanbase, but the most popular waifus are Rosalina and Palutena. Peach, Lucina, Samus, and Robin are also quite popular (also Isabelle, for some reason). Some people use the term waifu as a joke, using characters that people wouldn't usually be attracted to (i.e., "Navi is my waifu").
Some people joke that a male is their waifu. (i.e., "Wario is my waifu.").