Elision occurs before a vowel and a silent h (mute h). In words: ce, de, je, le, me, ne, que, se, te the final e is dropped and replaced with an apostrophe:
j'aime
je n'aime pas
je t'aime
The word la loses the vowel a : la + usine > l'usine.
The word si loses the vowel i before the pronouns il and ils.