The Spanish subjunctive mood is one of the most difficult concepts for English speakers to learn. While English has a subjunctive mood, its grammatical formulations aren't quite as different from the indicative tenses.

You use the subjunctive for various reasons:

cases of desire
Espero que vengan.
I hope that they come.

to express doubt
Dudo que vayas.
I doubt that you go.

to make impersonal statements
Que bueno que se apoye.
How good that it is supported.

to talk about impeding events
Cuando vayas manda mis saludos.
When you go, say "hi" for me.

Spanish also has a past subjunctive that describes past events in the same way as above.