What is the difference between 'em and them?
I saw my friend writing,
But I don't know what it means and if it is correct to use. Could you help me?
It's a contracted, or shortened, form of "them" that's used in conversation, much in the same way that "gonna" is often used to mean "going to". In your quote, there should be a space, so: "Let's kick 'em." The apostrophe is to show a contraction.
So yes, it's correct to use, and often used in conversation. However, if you are writing it, it should only be used to directly quote someone who has said it.
First of all, 'em can mean him, her or them (as a direct or indirect object).
As such, it is not actually a contraction or abbreviation of "them."
The 'em is an oral survival of the Old English dative pronoun him, either singular or plural. Contrary to what you may have learned elsewhere, it is not a contraction of them.
See also the OED:
Originally the dative plural third person personal pronoun form, cognate with Old Frisian him
As a so-called "oral survivor," it is best used only in informal speech, either spoken or written.