When we shorten words we use an apostrophe to show that letters have been missed out.
For example, do not becomes don't, and the apostrophe shows that the 'o' has been missed out of the word not.
We often run words into each other and shorten them when we use pronouns (words like I, you, he, she and it) and verbs.
I'm going out soon.
He's leaving tonight.
We're having a party tomorrow.