Mood refers to special verb forms that convey information about the speaker's desires, certainty, or opinion regarding a fact. The main categories for mood in Spanish are the indicative, the majority of verb forms you have seen up till now, e.g.: Ya me voy; the subjunctive, e.g.: Quiero que tú te vayas; and the conditional, e.g.: Me gustaría irme.
The imperative and the interrogative also fall into the category of mood.
Like tense, information regarding mood is indicated on the verb by stem changes and suffixes.