What is the subjunctive?

The subjunctive is a mood which is usually used to express uncertainty, doubt and feelings as opposed to the indicative mood which is used to demonstrate certainty. For example, je sais que Charles va au restaurant. I know that Charles is going to the restaurant. The verb aller is in the indicative as you know, and can say for sure, that Charles is going to the restaurant. However, je ne crois pas que Charles aille au restaurant. As you do not know whether or not Charles is going to the restaurant, the verb aller is constructed using the subjunctive.