Modern Greek: Quite often. They are remnants of earlier distinctions, and, through centuries upon centuries of sound change, now simply indicate the stressed syllable. The syllable that carries it should be stressed.
Ancient Greek (generally speaking): Yes, along with several other diacritics. (technical term for such "signals") I believe they indicated pitch accent, and were rather unrelated to stress. I think it actually came to indicate stress today due to some scholars assuming Greek must be stress accented like Latin.
Hope that helps. If you really want to know more, Wikipedia has some very informative articles on the subject.
I typed the word "γραμμα" in a translator and got "program" as an answer, i realized i hadn't added the accute accent (´), so i wrote γράμμα and the translator gave me the answer "letter".
So now I know how important the accute accent is in the greek language! As Sean stated above "It can change the meaning of a word completely." And boy it does!
The letter being taught here is named "yiota" but you don't need to worry about that. The letter in Greek looks like this:
Ιι that's right it looks like a Ii (but without the dot on the small letter), and it is pronounced like "ee" in "sEE", "bE" etc.
See here for how to get the keyboard and how the letters are pronounced, where to find them on the keyboard etc.
HOW TO GET THE GREEK KEYBOARD https://forum.duolingo.com/comment/17486118
These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it, how to add accents etc.