In French the subjunctive is often described as a mood tense, it expresses things that are subjective; moods, feelings and emotions. The subjunctive is often introduced by 'que' , for example J'ai peur que, or Il faut que. 

A good example of when to use the subjunctive can be seen in the phrase 'Il semble que' and 'Il me semble que'. You use the subjunctive with the phrase 'Il semble que' because 'it seems that' is an uncertain phrase. The phrase 'Il me semble que' though uses the regular tense because 'it seems to me that' is a certain phrase. It is sure that you think this.