Well, I must admit that french grammar is not really easy. That's why I admire those who try to learn it !
Firstly, the Subjontcif is not a tense, it is a mode gathering fort tenses : le subjonctif présent, le subjonctif passé, le subjonctif imparfait and le subjontcif plus-que-parfait.
It expresses desire, wish, a imaginary fact.
Example : Il semble que je ne puisse pas le faire ( opinion )
It seems that I can't do it
But relax : we don't often use it and I think that only few people correctly use it.