The subjunctive is what we call a verb mood. There are basically 3 moods: INDICATIVE, SUBJUNCTIVE, and IMPERATIVE.
Indicative - most sentences. This states facts and asks questions. (Don't get too caught up on the word "fact" here. It's not a matter of the truth/fiction of the information.)
Subjunctive - this is used to express wishes, desires, doubt, uncertainty. You can think of it as expressing things that the speaker is more subjective about.
Imperative - commands.
Each MOOD has TENSES under it. If you're using the Verb Fill-in chart I gave you back in Semana 2, you’ll see the list of tenses under each mood.