You know how, when you turn could have into could’ve, you use an apostrophe? Ever wonder why that is?
The word ’em is shortened from them; the beginning is missing, so you need an apostrophe to show where the missing letters were, just as the apostrophe in could’ve shows where the first letters of have are missing. (If you want the jargon, this is called an elision.)
“Don’t play games with a girl who can play ’em better.”
(WordPress’ spell checker thinks could’ve is not a word. Mercenary Proofreader thinks that’s funny.)