Subjonctive is a verb tense that I use in French when you would use infinitive.
For example, you'd say "I want you to sing a song". [to sing = infinitive].
I would say " Je veux que tu chantes une chanson" [que tu chantes = subjonctive]
Subjonctive looks like other verb tense (present for the verb finishing in ER) so yopu probably used it or saw it already without noticing it.
Subjonctive is used in many cases, but most commonly with expressions like "j'aimerais que", "il faut que", etc. The important word is "que" + pronoun.