bad title?

Home is where the heart is.

Yeah, I guess it is. There are several weak attempts I can make at explaining it, but they all leave something wanting.

I will iterate these attempts, now:

Home is getting Spock back in his right mind.
Home is returning to their present, after a trip to the past.
Home is a trip to the 20th century, where the cast, crew and audience call home.
Home is the crew being reinstated after being grounded.
Home is the new Enterprise.
Home is where they're headed at the start, but they must detour through the past to save the present before they get there.

That last one is perhaps the closest to satisfactory... But only if the title were sightly amended to: A Funny Thing Happened on the Voyage Home from Vulcan.

Back to the Future would have been better but Doc Brown died in The Search for Spock.

They're all humans (even Spock if you want to get technical about it) and all but Spock are Terrans ("earthlings").

So seeing as they begin the movie starting their trip back to Earth, and most of the movie takes place in 1986 past Earth, the audience presumption is that "The Voyage" refers to a time travel adventure to save and return "Home," or 23rd century Earth.

There's a twist at the end, though, remember.

Kirk: "My friends, we've come home."

It turns out the home they were voyagin to was the reborn Enterprise, even if they didn't know it until just then.

Back to the future.