My interpretation of the trailer is fairly strait forward. There are new faces suggesting that the core group have fortified their numbers. This is most likely accomplished with the former Woodbury population since this lends continuity with respect to where they left off the season.
They have modified the prison somewhat but despite this they keep losing people. They are most likely under attack by external covert forces and possibly internal as they may have taken in some people loyal to the Governor.
The accumulation of walkers at specific points around the outer fence as well dead rats at said points and dead animals found further out suggest that walkers are being lead to the prison,
.by someone who aims to weaponize them against the group.
This is likely someone that has a grudge possibly the Governor since he no longer has the numbers to attack openly.
Its probably not Morgan (Lennie James) but since he is out of his head its not impossible.
It looked like someone was breaking into the prison or areas of the prison judging from the way Rick was examining doors / doorways.
The opening scene at the "Big Spot" super store was likely just a supply run (baby formula, meds and stuff).
In the books there was this great part where Tyreese was trapped in a room with a large number of Walkers and just a hammer. IIRC this was during the initial push to clear the prison and they all thought he was dead. When they went to retrieve his body they found that he had held his own and was still alive. In the trailer though it doesn't look good for Tyreese being surrounded by all those walkers like that.
At the end of the trailer the radio massage, in a woman's voice, was as follows:
Those who arrive,