What is the meaning of S'?

Clitic form of it's.

Clitic form of so.

Clitic form of is.

Clitic form of see; used in s'you later.

negates the meaning of the modified verb: not, don't

apocopic form of se before a vowel

contraction of es

apocopic form of se

apocopic form of sè

his; her; its (elided form of son or sa before a word starting with a vowel)

apocopic form of se (“if”)

apocopic form of si, used before a vowel

Elision of si (“if”) before il or ils.

Elision of se before a word beginning with a vowel.

Elision of se before a word beginning with a consonant.

apocopic form of si

Present/future copula form

Used to introduce the comparative/superlative form of adjectives

apocopic form of se

third-person singular reflexive pronoun; oneself

apocopic form of su, used before a vowel: the (masculine singular definite article)

apocopic form of sa, used before a vowel: the (feminine singular definite article)

Source: wiktionary.org