Short for them, especially in informal use.let 'em know who’s boss...
The others all put on good show so if you like 'em you'll probably see 'em.
Plenty of American films get made without a single British villain in 'em.
Allowing for time difference James is about to hit the Broadway stage now so all I can say is knock 'em dead James!
Origin
Middle English: originally a form of hem, dative and accusative third person plural pronoun in Middle English; now regarded as an abbreviation of them.