When you want to say "I agree", why do you say "Ich stimme zu" instead of just "Ich stimme"? What part of speech is zu? What does it mean here? Is it more common to say "Ich stimme zu" instead of just "Ich stimme"?
I did look up this word on Wiktionary, and found that it is most likely an adverb in this context. But it is hard for me to determine the meaning of this word. Does it mean "too"? That is my best guess... But I still don't see how it fits in with the sentence.