I'm not a teacher, but I will give you a very short explanation (too short).
Indicativo : real actions, real facts -----> yo hablo español = I speak Spanisch (that's true)
Subjuntivo : not real actions, wishes, probabilities, … -----> si yo hablara español = if I spoke Russian (I don't speak Russian but maybe I would like to speak someday)
And that's only the beginning :)) I hope, it helps you a little bit and I wish you the best with this language.