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, Robin, Callie and Marie 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 person is their waifu (despite the presense of a male version to said term). (i.e., "Waluigi is my waifu.").