Decidió is the conjugation of the verb decidir for the third person singular in the preterite tense. The preterite 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. So this sentence means that when the child heard the bear he decided to run away from the forest. Finally:
Cuando el niño oyó al oso decidió huir del bosque
