In addition, like most Slavic languages, the Shtokavian verb also has one of two aspects: perfective or imperfective. Most verbs come in pairs, with the perfective verb being created out of the imperfective by adding a prefix or making a stem change. The imperfective aspect typically indicates that the action is unfinished, in progress, or repetitive; while the perfective aspect typically denotes that the action was completed, instantaneous, or of limited duration. Some Štokavian tenses (namely, aorist and imperfect) favor a particular aspect (but they are rarer or absent in Čakavian and Kajkavian). Actually, aspects "compensate" for the relative lack of tenses, because aspect of the verb determines whether the act is completed or in progress in the referred time.
What is the stem change created out of?