Depending on it’s context, after all might be an open form compound word (two words that are taken together to form a single word form, but without a hyphen and without one being rammed up the backside of the other word without a space between them, if you’ll excuse the ribald imagery).
In this case, after all means one of two things. For one, it could mean, “after considering everything.” An example of the use of this sense of after all is, “He seemed like he was smart because he was such a great bullshitter, but he turned out to be an idiot after all.”
The second meaning of after all is along the lines of, “you’ll have to excuse them because, well, you know, they are who they are and they can’t help themselves.”
That’s a little long-winded. An example might help. For instance, you might say, “Sure he’s heartless, but what do you expect? After all, he’s only a Republican.”
I suspect it’s only Democrats, Independents and much of the world outside of the United States who would say that.
In some contexts the after and all may be separate words rather than forming a compound word of any sort. A common example of this is, “After all is said and done …” But that’s become such a cliché that no one would actually say that, would they? I mean, other than me.