Pronunciation of 妨碍 (fáng'ài) For any pronunciation the tone is on the vowel in sequence of a, e, i, o,u which appears first.If it is a two character  word, there will not be any symbol between two characters.
But in 妨碍 (fáng'ài), what is the character between fáng and ài. What differentiation it makes with this character “'”?
It is called 隔音符號 (lit. symbol for sound segregation). When you group pinyin syllables by word, sometimes confusion may arise. With fángài, do you mean fán gài or fáng ài?

An apostrophe is needed before syllables that begin with a, o, or e to avoid confusion (because they have a silent initial, i.e. 零聲母). So if you write fángài, you only mean fán gài (there is no word for this), but if you write fáng'ài, you mean fáng ài 妨礙.

Other examples include: Xī'ān 西安 vs xiān 先, pí'ǎo 皮襖 vs the syllable piao.


Reference: 漢語拼音方案