The plot point in question is twofold, with one half being pretty obvious (since they gave it more than a second of onscreen time and a line of dialogue to call attention to it), the second half less so.
So, potential spoilers ahead for those who don't like that sort of thing.
"What is this place?" Asks Andrea at one point in the ad, after she and Michonne have been accosted in the woods at gunpoint. It's clear, especially from the use of The Governor in a shot just before that one in the trailer, that the answer is Woodbury, and that Michonne and Andrea will spend some time there early in the season.
That's not totally surprising, since there has been at least one blink-and-you-miss-it shot of the two of them in Woodbury in a previous trailer.
With the buzz cut, the khaki and the fact that he's holding the gun with his left hand, we're pretty sure that's Merle Dixon.
That might explain why Laurie Holden, who plays Andrea in the show, is looking just this side of hysterical in both of the screen shots we've taken here--and why Danai Gurira's Michonne doesn't really seem to understand in the photo at left exactly what's got her so worked up.
For Merle to come across someone from the group, but not his brother, right away could put things between the survivors and The Governor's encampment at Woodbury off on the wrong foot in a big, immediate way that didn't (couldn't) happen in the comic books. It might go some ways toward humanizing The Governor somewhat, making him a more believable antagonist rather than just a moustache-twirling villain, which is something that the showrunners have expressed an interest in doing. After all, if he's got someone in his ear telling him that these people are dangerous and deceitful, acting out against them is more in the interest of his people than any motivation he ever had in the source material.
And that jives with earlier comments that there would be a major break from the source material early in the season. Whether it was in the premiere or just early in the season we can't recall, but those comments had led many people to believe that there might be an early, unexpected death (Glenn being one of the favorite choices for that, since his character recently died in the comics) but what if the changes were less emotionally loaded than that and more exactly what was teased--changes?