I'm not sure what you mean. An apostrophe is a punctuation mark ('). It is used to
- show possession - David's bicycle ( the bicycle belongs to David)
- show a relationship - Sarah's brother
- replace missing letters in contractions - he's (he is), isn't (is not)
- indicate missing letters when dialect or accent is put into writing - 'em (them), 'im (him), 'er (her), etc.