As an example, the Hebrew word for if can be written in the English alphabet as
’im. What does the apostrophe represent here?
Transliteration conventions are numerous, so there are many possible meanings.
Often apostrophe semi-formally represents some character that does not exist in the Latin alphabet. In some such cases apostrophe itself is an ASCII substitute for a transalphabetic character.
You can find the following uses:
From Cyrillic: soft sign ь
From Armenian, Georgian, Indic etc: aspiration of the preceding consonants p, t or k
From Semitic scripts: ayn, glottal stop
Across a few alphabets as in the Latin one, it is used to represent a preceding acute or grave accent or stress mark, and of course elision.