The subjunctive is a mood. When a speaker or writer expresses their feelings about what they are saying/writing. For example: 1) I don't think my boyfriend loves me. Vs. 2) I know my boyfriend doesn't love me.
In sentence 1, we can see that the speaker expresses uncertainty about whether or not the boyfriend loves her. In sentence 2, the speaker is sure or certain that the boyfriend doesn't love her.
In English, the verbal tense (doesn't love me) doesn't change, but in Spanish, the tense in sentence 1 (the one with uncertainty), the tense chages from the present to the present subjunctive: No creo que mi novio me ame (not "que mi novio me ama"). However, the second sentence would not be subjunctive because the speaker is certain of what she is saying (Se que mi novio no me ama).