The word fue is the conjugation of the verb ir for the third person singular in the preterite tense. So this tense is one of two simple past tenses. It is used to describe actions that took place or were completed at a certain point in the past. This sentence is false because it stands for the preterite tense but the last word is mañana that means tomorrow, hence it's illogical that you use this tense along with the word mañana.
