Recently, I've heard that someone said "get him back". I'm not sure about the exact meaning though I can guess some. Could you please explain the meaning?
He went away and now we need him again, so we would say to someone "Get him back". I understand it as a short version of "Get him (Ask him, Compel him, Make him, Fetch him ...) (... to come) back (to here)". It is very short but that is how it is usually understood.
As Anton comments, "Get him back" can mean to make someone return.
But there is another meaning. If you "get someone back", you could be getting revenge on them. They "got" you and you "got" them back".
get someone back: to do something unpleasant to someone because they have done something unpleasant to you:
I'll get you back for this, just you wait!
I think he's trying to get back at her for those remarks she made in the meeting. - Cambridge Dictionaries
You might also consider Noah's comment, which is summed up by Wiktionary:
To be prepared and willing to support or defend (someone).
Going a little further afield, to get someone's back up is to annoy them:
get (or put) someone's back up
Definition of get or put someone's back up in English:
Make someone annoyed or angry. - Oxford Dictionaries
Complicated, isn't it? You have to rely on context to sort some of this out.
This can be interpreted several different ways.
If a big boss tells his secretary "get him back", that means she should attempt to reach "him" on the phone (or some other medium obvious from the context). Ie, reestablish contact with "him".
If you hear a rough looking character say that he going to "get him back", referring to some other individual, it generally means that the first character intends to retaliate against the second character for some real or perceived slight or injury.
If, on the other hand, you hear someone say that Character One "has the back" or "has got the back" of Character Two, that means that Character One is protecting Character Two (in effect, standing behind him to make sure on one sneaks up on him from behind).
You might also hear someone say "Did the computer guy leave yet?" (Response "Yes") "Well, get him back (here), we still have a problem."
This is one of those cases where context is very important.
To "get (someone) back" means to return the same that they did (or gave) to you. If they wronged you, you'd try to get revenge. If they loaned you twenty bucks, you might say "Thanks - I'll get you back," meaning: you'd return the favor.