Wet blanket alert: pedant at large!
Commonly, when singing in French, one pronounces weak syllables that are normally silent (elided) in normal speech. Thus, jeune(s) is normally spoken as one syllable, but is most frequently sung as two: jeu-ne(s). (Sung Spanish has an opposite convention: one normally combines adjacent vowels--of whatever quality--into a single syllable, and singing them separately is contrary to normal expectations.) Conversely, weak syllables which in speech are pronounced--however fleetingly--may be completely elided when sung. If you see the word "matelot" (sailor) in a text, it might be sung as either three or two syllables; both are common.

What's more, technically French doesn't make marked differences in stress between syllables, so metric stress and length often fall on weaker syllables. In fact, you'll even find metric stress on words like "le" (the), "un/une" (a) and "de/des" (of), whose English counterparts are rarely stressed. Considering this great flexibility, it can be difficult to predict how a text as most commonly sung aligns with the music. So, as an aid to singers, many people explicitly indicate all or most elisions with apostrophes. You won't properly appreciate the utility of this until you compare your best guesses to recordings.