In Spanish, the subjunctive mood is used to communicate meanings which are subjective or less certain than the indicative mood. 

The indicative mood is used to express certainty and objectivity; whereas the subjunctive mood expresses doubt, hopes, fears and uncertainty etc. 

Example: 

I doubt that he will go to the beach. 

Dudo que vaya a la playa. 

I hope Julie will return soon. 

Espero que Julia vuelva pronto.