As Jon says, 'em' is a kind of shortened form of the pronoun 'them'. Tor But please note that 'em is not a standard abbreviation, like 'isn't' or 'I'm' or 'doesn't' or 'hasn't'. Words like 'isn't' are part of normal, correct, informal written English - 'em is NOT. The form 'em' is not something which we normally write. It's a sound, not a word. It's an imitation of the weak 'um' sound which naturally replaces the pronoun 'them' in some forms of rapid, casual native-speaker speech. Don't write it, and don't consciously try to say it. By the way, 'em is never capitalised, because it always occurs in the middle of a sentence. We very rarely write 'em', but in the rare cases when we do write it - as a way of indicating informal speech - it's always attached to the previous word, for example, 'tell'em' or 'give'em'. If you write it with a capital 'E' it becomes a name. 'Em' with a capital letter is the short form of an name such as Emma or Emily. 'Tell Em it's my birthday' would mean 'Tell my friend Emily or Emma that it's my birthday.'

"'em" is an abbreviation of them. So they are saying, "tell them it's* (it is) my birthday."