'Em is the same as the pronoun them; it is never used in stressed positions and it is somewhat colloquial, such that you will often find it in popular music but not in scientific articles.
Although it may seem so, 'em is not an abbreviation of them. These words have different origins: 'em comes from Old English whereas them comes from Old Norse.
'em is the short form of the word "them".
According to en.oxforddictionaries.com' 'em is defined as:
‘let 'em know who's boss’
short for them, especially in informal use
It has also a mention about its origin: 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.
The sentence posted in question may be expanded as "So smoke them if you got them" (though we are not clear about the context)
Hope it helps.