Sometimes orthographic accents are used to distinguish two related words depending on how they function in a sentence. For example, este means "this" as an adjective while éste means "this" as a pronoun. (In modern usage, such an orthographic accent is sometimes omitted if the sentence is unambiguous without out it.)
In other instances, an orthographic accent is used to distinguish two unrelated words. For example, se is a reflexive pronoun meaning "himself," "herself," "itself," or "themselves," while sé is a conjugated verb meaning either "I know" (from saber) or "you be" (from ser).
Some writers also use an accent over the o ("or") when it comes between two numerals so it is not confused with a 0 (zero). Example: Murieron 8 ó 9 personas. (8 or 9 people died.)
Note that in all these examples, the orthographic accent doesn't affect which syllable of the word is stressed.