For example, instead of saying: "I am excited for summer", we could say, "I'm excited for summer." We shorten two words ( I am) into one word (I'm).
We do this by replacing the missing letter by an apostrophe
I am = I'm
As you can see, we substitute the 'a' with apostrophe mark. You'll notice the same thing from all the contractions below, all the missing letters are replaced with an apostrophe.
(Did you notice how many contractions I used in the last sentence?)