An accent marks a given syllable as the stressed syllable. video Accent marks are not widely used in English.
Greek:
Greek has three different accent marks (BBG 4.2.3):
ά – acute
ᾶ – circumflex (often a sign that a contraction happened)
ὰ – grave
For much more, see Lesson 2 in Machen’s grammar.
Hebrew:
Most words are accented on the last syllable. If this is not the case, then an accent is used to mark the stressed syllable; see this. If you like, here is a detailed presentation on all the Hebrew accents. (not necessary to be learned in this class)