Contractions are the words that are formed when certain letters are left out of a word or phrase, usually taking the form of single words that replace two (or sometimes more) words. The letters that are left out are replaced with an apostrophe. Familiar forms of contractions include a helper verb + ‘not’ (the phrase “do not” becomes don’t; “have not” becomes haven’t, etc.), a personal pronoun + ‘to have’ (the phrase “I have” becomes I’ve, etc.), and a helper verb plus the a form of ‘to be’ (the phrase “you are” becomes you’re, etc.).
Note especially the word it’s, which is a contraction of the phrase “it is”. The word it’s is not the possessive form of it. [The possessive of the word it is formed without an apostrophe (its) and is therefore an exception to the rule that singular possessives are formed by adding ‘s to the end of a word.]
Contractions are considered informal and are therefore generally avoided in academic texts.